feat(mail): resizable columns, account in folder tree, mail settings in CRM settings

- Add ResizablePanel component (components/ui/ResizablePanel.tsx) with
  drag-to-resize via mouse events, no external dependencies
- Replace fixed-width columns in Mail.tsx with ResizablePanel for all
  three panes (folder tree, mail list, mail detail)
- Remove account-select toolbar item from Mail.tsx; account selection
  is now integrated into the folder tree
- Restructure MailFolderTree to show mail accounts as top-level nodes
  with folders nested underneath, built from flat list via parent_id
- Add expand/collapse chevrons for accounts and folders with children
- Load folders for ALL accounts (not just selected one) so multi-account
  tree works
- Derive selectedAccountId from selected folder instead of separate
  account selector
- Add Mail tab to CRM Settings page (Settings.tsx) with 📧 icon
- Add /settings/mail route pointing to MailSettingsPage
- Update settings link in mail toolbar to point to /settings/mail
- Update no-accounts empty state link to /settings/mail

frontend/src/pages/Mail.tsx

@@ -1,5 +1,7 @@
 /**
  * Mail page — folder tree + mail list + reading pane + compose.
+ * Uses ResizablePanel for drag-to-resize columns.
+ * Account selection is integrated into the folder tree (left pane).
  */
 
 import React, { useState, useEffect, useCallback } from 'react';
@@ -9,6 +11,7 @@ import { Card } from '@/components/ui/Card';
 import { Button } from '@/components/ui/Button';
 import { useToast } from '@/components/ui/Toast';
 import { EmptyState } from '@/components/ui/EmptyState';
+import { ResizablePanel } from '@/components/ui/ResizablePanel';
 import { MailFolderTree } from '@/components/mail/MailFolderTree';
 import { MailList } from '@/components/mail/MailList';
 import { MailDetail } from '@/components/mail/MailDetail';
@@ -89,7 +92,7 @@ export function MailPage() {
       const sigs = await fetchSignatures();
       setSignatures(sigs);
     } catch {
- // non-critical
+      // non-critical
     }
   }, []);
 
@@ -98,27 +101,35 @@ export function MailPage() {
     loadSignatures();
   }, [loadAccounts, loadSignatures]);
 
-  // Load folders when account changes
-  const loadFolders = useCallback(async () => {
-    if (!selectedAccountId) return;
+  // Load folders for ALL accounts
+  const loadAllFolders = useCallback(async () => {
+    if (accounts.length === 0) return;
     setLoadingFolders(true);
     try {
-      const folderList = await fetchFolders(selectedAccountId);
-      setFolders(folderList);
-      // Auto-select first folder
-      if (folderList.length > 0 && !selectedFolderId) {
-        setSelectedFolderId(folderList[0].id);
+      const allFolders: MailFolder[] = [];
+      for (const acc of accounts) {
+        const folderList = await fetchFolders(acc.id);
+        allFolders.push(...folderList);
       }
+      setFolders(allFolders);
     } catch (err) {
       const msg = err instanceof Error ? err.message : String(err);
       setError(msg);
     }
     setLoadingFolders(false);
-  }, [selectedAccountId, selectedFolderId]);
+  }, [accounts]);
 
   useEffect(() => {
-    loadFolders();
-  }, [loadFolders]);
+    loadAllFolders();
+  }, [loadAllFolders]);
+
+  // Auto-select first folder when folders are loaded
+  useEffect(() => {
+    if (folders.length > 0 && !selectedFolderId) {
+      setSelectedFolderId(folders[0].id);
+      setSelectedAccountId(folders[0].account_id);
+    }
+  }, [folders, selectedFolderId]);
 
   // Load mails when folder or page changes
   const loadMails = useCallback(async () => {
@@ -146,32 +157,42 @@ export function MailPage() {
     loadMails();
   }, [loadMails]);
 
-  // Handle folder selection
-  const handleSelectFolder = useCallback((folderId: string) => {
-    setSelectedFolderId(folderId);
-    setMailsPage(1);
-    setSearchQuery('');
-    setSelectedMail(null);
-  }, []);
+  // Handle folder selection — derive account from folder
+  const handleSelectFolder = useCallback(
+    (folderId: string) => {
+      setSelectedFolderId(folderId);
+      const folder = folders.find((f) => f.id === folderId);
+      if (folder) {
+        setSelectedAccountId(folder.account_id);
+      }
+      setMailsPage(1);
+      setSearchQuery('');
+      setSelectedMail(null);
+    },
+    [folders],
+  );
 
   // Handle mail selection
-  const handleSelectMail = useCallback(async (mail: Mail) => {
-    setLoadingMail(true);
-    try {
-      const detail = await getMail(mail.id);
-      setSelectedMail(detail);
-      // Mark as seen if not seen
-      if (!detail.is_seen) {
-        await updateFlags(mail.id, { seen: true });
-        setMails((prev) => prev.map((m) => (m.id === mail.id ? { ...m, is_seen: true } : m)));
+  const handleSelectMail = useCallback(
+    async (mail: Mail) => {
+      setLoadingMail(true);
+      try {
+        const detail = await getMail(mail.id);
+        setSelectedMail(detail);
+        // Mark as seen if not seen
+        if (!detail.is_seen) {
+          await updateFlags(mail.id, { seen: true });
+          setMails((prev) => prev.map((m) => (m.id === mail.id ? { ...m, is_seen: true } : m)));
+        }
+      } catch (err) {
+        const msg = err instanceof Error ? err.message : String(err);
+        toast.error(msg);
+        setSelectedMail(mail);
       }
-    } catch (err) {
-      const msg = err instanceof Error ? err.message : String(err);
-      toast.error(msg);
-      setSelectedMail(mail);
-    }
-    setLoadingMail(false);
-  }, [toast]);
+      setLoadingMail(false);
+    },
+    [toast],
+  );
 
   // Handle compose
   const handleCompose = useCallback(() => {
@@ -198,72 +219,84 @@ export function MailPage() {
   }, []);
 
   // Handle toggle flag
-  const handleToggleFlag = useCallback(async (mail: Mail) => {
-    try {
-      await updateFlags(mail.id, { flagged: !mail.is_flagged });
-      setSelectedMail((prev) => prev ? { ...prev, is_flagged: !mail.is_flagged } : prev);
-      setMails((prev) => prev.map((m) => (m.id === mail.id ? { ...m, is_flagged: !mail.is_flagged } : m)));
-    } catch (err) {
-      toast.error(err instanceof Error ? err.message : String(err));
-    }
-  }, [toast]);
+  const handleToggleFlag = useCallback(
+    async (mail: Mail) => {
+      try {
+        await updateFlags(mail.id, { flagged: !mail.is_flagged });
+        setSelectedMail((prev) => (prev ? { ...prev, is_flagged: !mail.is_flagged } : prev));
+        setMails((prev) => prev.map((m) => (m.id === mail.id ? { ...m, is_flagged: !mail.is_flagged } : m)));
+      } catch (err) {
+        toast.error(err instanceof Error ? err.message : String(err));
+      }
+    },
+    [toast],
+  );
 
   // Handle create event from mail
-  const handleCreateEvent = useCallback(async (mail: Mail) => {
-    const title = mail.subject || t('mail.noSubject');
-    const now = new Date();
-    const start = now.toISOString();
-    const end = new Date(now.getTime() + 60 * 60 * 1000).toISOString();
-    const payload: CreateEventFromMailPayload = {
-      title,
-      start,
-      end,
-      description: mail.body_text.slice(0, 500),
-    };
-    try {
-      await createEventFromMail(mail.id, payload);
-      toast.success(t('mail.eventCreated'));
-    } catch (err) {
-      toast.error(err instanceof Error ? err.message : String(err));
-    }
-  }, [toast, t]);
+  const handleCreateEvent = useCallback(
+    async (mail: Mail) => {
+      const title = mail.subject || t('mail.noSubject');
+      const now = new Date();
+      const start = now.toISOString();
+      const end = new Date(now.getTime() + 60 * 60 * 1000).toISOString();
+      const payload: CreateEventFromMailPayload = {
+        title,
+        start,
+        end,
+        description: mail.body_text.slice(0, 500),
+      };
+      try {
+        await createEventFromMail(mail.id, payload);
+        toast.success(t('mail.eventCreated'));
+      } catch (err) {
+        toast.error(err instanceof Error ? err.message : String(err));
+      }
+    },
+    [toast, t],
+  );
 
   // Handle attachment download
-  const handleDownloadAttachment = useCallback(async (mailId: string, attachment: MailAttachment) => {
-    setDownloadingAttachmentId(attachment.id);
-    try {
-      const blob = await downloadAttachment(mailId, attachment.id);
-      const url = window.URL.createObjectURL(blob);
-      const a = window.document.createElement('a');
-      a.href = url;
-      a.download = attachment.filename;
-      window.document.body.appendChild(a);
-      a.click();
-      window.document.body.removeChild(a);
-      window.URL.revokeObjectURL(url);
-    } catch (err) {
-      toast.error(err instanceof Error ? err.message : String(err));
-    } finally {
-      setDownloadingAttachmentId(null);
-    }
-  }, [toast]);
+  const handleDownloadAttachment = useCallback(
+    async (mailId: string, attachment: MailAttachment) => {
+      setDownloadingAttachmentId(attachment.id);
+      try {
+        const blob = await downloadAttachment(mailId, attachment.id);
+        const url = window.URL.createObjectURL(blob);
+        const a = window.document.createElement('a');
+        a.href = url;
+        a.download = attachment.filename;
+        window.document.body.appendChild(a);
+        a.click();
+        window.document.body.removeChild(a);
+        window.URL.revokeObjectURL(url);
+      } catch (err) {
+        toast.error(err instanceof Error ? err.message : String(err));
+      } finally {
+        setDownloadingAttachmentId(null);
+      }
+    },
+    [toast],
+  );
 
   // Handle send (compose)
-  const handleSend = useCallback(async (payload: SendMailPayload | ReplyPayload | ForwardPayload, mode: ComposeMode) => {
-    try {
-      if (mode === 'reply' && replyToMail) {
-        await replyMail(replyToMail.id, payload as ReplyPayload);
-      } else if (mode === 'forward' && forwardMailState) {
-        await forwardMail(forwardMailState.id, payload as ForwardPayload);
-      } else {
-        await sendMail(payload as SendMailPayload);
+  const handleSend = useCallback(
+    async (payload: SendMailPayload | ReplyPayload | ForwardPayload, mode: ComposeMode) => {
+      try {
+        if (mode === 'reply' && replyToMail) {
+          await replyMail(replyToMail.id, payload as ReplyPayload);
+        } else if (mode === 'forward' && forwardMailState) {
+          await forwardMail(forwardMailState.id, payload as ForwardPayload);
+        } else {
+          await sendMail(payload as SendMailPayload);
+        }
+        toast.success(t('mail.sent'));
+      } catch (err) {
+        toast.error(err instanceof Error ? err.message : String(err));
+        throw err;
       }
-      toast.success(t('mail.sent'));
-    } catch (err) {
-      toast.error(err instanceof Error ? err.message : String(err));
-      throw err;
-    }
-  }, [replyToMail, forwardMailState, toast, t]);
+    },
+    [replyToMail, forwardMailState, toast, t],
+  );
 
   // Handle search
   const handleSearch = useCallback((query: string) => {
@@ -271,17 +304,7 @@ export function MailPage() {
     setMailsPage(1);
   }, []);
 
-  // Handle account switch
-  const handleAccountSwitch = useCallback((accountId: string) => {
-    setSelectedAccountId(accountId);
-    setSelectedFolderId(null);
-    setSelectedMail(null);
-    setMails([]);
-    setMailsPage(1);
-    setSearchQuery('');
-  }, []);
-
-  // Register toolbar items
+  // Register toolbar items (account-select removed — account is now in folder tree)
   const registerItems = usePluginToolbarStore((s) => s.registerItems);
   const unregisterPlugin = usePluginToolbarStore((s) => s.unregisterPlugin);
   const selectedMailId = selectedMail?.id;
@@ -289,16 +312,6 @@ export function MailPage() {
   useEffect(() => {
     const hasMail = !!selectedMailId;
     registerItems('mail', [
-      {
-        id: 'account-select',
-        plugin: 'mail',
-        label: 'Account',
-        type: 'select',
-        group: 'account',
-        selectValue: selectedAccountId,
-        selectOptions: accounts.map((a) => ({ value: a.id, label: a.email })),
-        onSelect: handleAccountSwitch,
-      },
       {
         id: 'search',
         plugin: 'mail',
@@ -307,13 +320,18 @@ export function MailPage() {
         group: 'search',
         searchPlaceholder: 'E-Mails durchsuchen...',
         onSearch: handleSearch,
+        onClick: () => {},
       },
       {
         id: 'compose',
         plugin: 'mail',
         label: 'Verfassen',
         group: 'compose',
-        icon: <svg className="w-3.5 h-3.5" fill="none" viewBox="0 0 24 24" stroke="currentColor"><path strokeLinecap="round" strokeLinejoin="round" strokeWidth={2} d="M12 4v16m8-8H4" /></svg>,
+        icon: (
+          <svg className="w-3.5 h-3.5" fill="none" viewBox="0 0 24 24" stroke="currentColor">
+            <path strokeLinecap="round" strokeLinejoin="round" strokeWidth={2} d="M12 4v16m8-8H4" />
+          </svg>
+        ),
         onClick: handleCompose,
       },
       {
@@ -322,7 +340,11 @@ export function MailPage() {
         label: 'Antworten',
         group: 'mail-actions',
         disabled: !hasMail,
-        icon: <svg className="w-3.5 h-3.5" fill="none" viewBox="0 0 24 24" stroke="currentColor"><path strokeLinecap="round" strokeLinejoin="round" strokeWidth={2} d="M3 10h10a8 8 0 018 8v2M3 10l6 6m-6-6l6-6" /></svg>,
+        icon: (
+          <svg className="w-3.5 h-3.5" fill="none" viewBox="0 0 24 24" stroke="currentColor">
+            <path strokeLinecap="round" strokeLinejoin="round" strokeWidth={2} d="M3 10h10a8 8 0 018 8v2M3 10l6 6m-6-6l6-6" />
+          </svg>
+        ),
         onClick: () => selectedMail && handleReply(selectedMail),
       },
       {
@@ -331,7 +353,11 @@ export function MailPage() {
         label: 'Weiterleiten',
         group: 'mail-actions',
         disabled: !hasMail,
-        icon: <svg className="w-3.5 h-3.5" fill="none" viewBox="0 0 24 24" stroke="currentColor"><path strokeLinecap="round" strokeLinejoin="round" strokeWidth={2} d="M21 10H11a8 8 0 00-8 8v2m18-10l-6 6m6-6l-6-6" /></svg>,
+        icon: (
+          <svg className="w-3.5 h-3.5" fill="none" viewBox="0 0 24 24" stroke="currentColor">
+            <path strokeLinecap="round" strokeLinejoin="round" strokeWidth={2} d="M21 10H11a8 8 0 00-8 8v2m18-10l-6 6m6-6l-6-6" />
+          </svg>
+        ),
         onClick: () => selectedMail && handleForward(selectedMail),
       },
       {
@@ -341,7 +367,11 @@ export function MailPage() {
         group: 'mail-actions',
         disabled: !hasMail,
         active: selectedMailFlagged,
-        icon: <svg className="w-3.5 h-3.5" fill="none" viewBox="0 0 24 24" stroke="currentColor"><path strokeLinecap="round" strokeLinejoin="round" strokeWidth={2} d="M3 21v-4m0 0V5a2 2 0 012-2h6.5l1 1H21l-3 6 3 6h-8.5l-1-1H5a2 2 0 00-2 2zm9-13.5V9" /></svg>,
+        icon: (
+          <svg className="w-3.5 h-3.5" fill="none" viewBox="0 0 24 24" stroke="currentColor">
+            <path strokeLinecap="round" strokeLinejoin="round" strokeWidth={2} d="M3 21v-4m0 0V5a2 2 0 012-2h6.5l1 1H21l-3 6 3 6h-8.5l-1-1H5a2 2 0 00-2 2zm9-13.5V9" />
+          </svg>
+        ),
         onClick: () => selectedMail && handleToggleFlag(selectedMail),
       },
       {
@@ -350,7 +380,11 @@ export function MailPage() {
         label: 'Termin',
         group: 'mail-actions',
         disabled: !hasMail,
-        icon: <svg className="w-3.5 h-3.5" fill="none" viewBox="0 0 24 24" stroke="currentColor"><path strokeLinecap="round" strokeLinejoin="round" strokeWidth={2} d="M8 7V3m8 4V3m-9 8h10M5 21h14a2 2 0 002-2V7a2 2 0 00-2-2H5a2 2 0 00-2 2v12a2 2 0 002 2z" /></svg>,
+        icon: (
+          <svg className="w-3.5 h-3.5" fill="none" viewBox="0 0 24 24" stroke="currentColor">
+            <path strokeLinecap="round" strokeLinejoin="round" strokeWidth={2} d="M8 7V3m8 4V3m-9 8h10M5 21h14a2 2 0 002-2V7a2 2 0 00-2-2H5a2 2 0 00-2 2v12a2 2 0 002 2z" />
+          </svg>
+        ),
         onClick: () => selectedMail && handleCreateEvent(selectedMail),
       },
       {
@@ -358,7 +392,11 @@ export function MailPage() {
         plugin: 'mail',
         label: 'Sync',
         group: 'tools',
-        icon: <svg className="w-3.5 h-3.5" fill="none" viewBox="0 0 24 24" stroke="currentColor"><path strokeLinecap="round" strokeLinejoin="round" strokeWidth={2} d="M4 4v5h.582m15.356 2A8.001 8.001 0 004.582 9m0 0H9m11 11v-5h-.581m0 0a8.003 8.003 0 01-15.357-2m15.357 2H15" /></svg>,
+        icon: (
+          <svg className="w-3.5 h-3.5" fill="none" viewBox="0 0 24 24" stroke="currentColor">
+            <path strokeLinecap="round" strokeLinejoin="round" strokeWidth={2} d="M4 4v5h.582m15.356 2A8.001 8.001 0 004.582 9m0 0H9m11 11v-5h-.581m0 0a8.003 8.003 0 01-15.357-2m15.357 2H15" />
+          </svg>
+        ),
         onClick: () => { /* sync trigger */ },
       },
       {
@@ -366,12 +404,19 @@ export function MailPage() {
         plugin: 'mail',
         label: 'Einstellungen',
         group: 'tools',
-        icon: <svg className="w-3.5 h-3.5" fill="none" viewBox="0 0 24 24" stroke="currentColor"><path strokeLinecap="round" strokeLinejoin="round" strokeWidth={2} d="M10.325 4.317c.426-1.756 2.924-1.756 3.35 0a1.724 1.724 0 002.573 1.066c1.543-.94 3.31.826 2.37 2.37a1.724 1.724 0 001.065 2.572c1.756.426 1.756 2.924 0 3.35a1.724 1.724 0 00-1.066 2.573c.94 1.543-.826 3.31-2.37 2.37a1.724 1.724 0 00-2.572 1.065c-.426 1.756-2.924 1.756-3.35 0a1.724 1.724 0 00-2.573-1.066c-1.543.94-3.31-.826-2.37-2.37a1.724 1.724 0 00-1.065-2.572c-1.756-.426-1.756-2.924 0-3.35a1.724 1.724 0 001.066-2.573c-.94-1.543.826-3.31 2.37-2.37.996.608 2.296.07 2.572-1.065z" /><path strokeLinecap="round" strokeLinejoin="round" strokeWidth={2} d="M15 12a3 3 0 11-6 0 3 3 0 016 0z" /></svg>,
-        onClick: () => window.location.href = '/mail/settings',
+        icon: (
+          <svg className="w-3.5 h-3.5" fill="none" viewBox="0 0 24 24" stroke="currentColor">
+            <path strokeLinecap="round" strokeLinejoin="round" strokeWidth={2} d="M10.325 4.317c.426-1.756 2.924-1.756 3.35 0a1.724 1.724 0 002.573 1.066c1.543-.94 3.31.826 2.37 2.37a1.724 1.724 0 001.065 2.572c1.756.426 1.756 2.924 0 3.35a1.724 1.724 0 00-1.066 2.573c.94 1.543-.826 3.31-2.37 2.37a1.724 1.724 0 00-2.572 1.065c-.426 1.756-2.924 1.756-3.35 0a1.724 1.724 0 00-2.573-1.066c-1.543.94-3.31-.826-2.37-2.37a1.724 1.724 0 00-1.065-2.572c-1.756-.426-1.756-2.924 0-3.35a1.724 1.724 0 001.066-2.573c-.94-1.543.826-3.31 2.37-2.37.996.608 2.296.07 2.572-1.065z" />
+            <path strokeLinecap="round" strokeLinejoin="round" strokeWidth={2} d="M15 12a3 3 0 11-6 0 3 3 0 016 0z" />
+          </svg>
+        ),
+        onClick: () => {
+          window.location.href = '/settings/mail';
+        },
       },
     ]);
     return () => unregisterPlugin('mail');
-  }, [accounts, selectedAccountId, selectedMailId, selectedMailFlagged, handleAccountSwitch, handleSearch, handleCompose, handleReply, handleForward, handleToggleFlag, handleCreateEvent]);
+  }, [selectedMailId, selectedMailFlagged, handleSearch, handleCompose, handleReply, handleForward, handleToggleFlag, handleCreateEvent]);
 
   if (loadingAccounts) {
     return (
@@ -391,7 +436,7 @@ export function MailPage() {
         <EmptyState
           title={t('mail.noAccounts')}
           description={t('mail.noAccountsDesc')}
-          action={<Link to="/mail/settings" className="text-primary-600 hover:text-primary-700">{t('mail.configureAccount')}</Link>}
+          action={<Link to="/settings/mail" className="text-primary-600 hover:text-primary-700">{t('mail.configureAccount')}</Link>}
         />
       </div>
     );
@@ -405,23 +450,36 @@ export function MailPage() {
         </div>
       )}
 
-      {/* Three-pane layout */}
+      {/* Three-pane layout with resizable panels */}
       <div className="flex flex-1 overflow-hidden">
-        {/* Folder tree */}
-        <div className="w-56 border-r border-secondary-200 overflow-y-auto bg-white" data-testid="mail-folder-pane">
+        {/* Folder tree — resizable */}
+        <ResizablePanel
+          initialWidth={224}
+          minWidth={150}
+          maxWidth={400}
+          className="border-r border-secondary-200 overflow-y-auto bg-white"
+          data-testid="mail-folder-pane"
+        >
           <div className="p-3">
             <h3 className="text-xs font-semibold text-secondary-500 uppercase mb-2">{t('mail.folders')}</h3>
             <MailFolderTree
+              accounts={accounts}
               folders={folders}
               selectedFolderId={selectedFolderId}
               onSelect={handleSelectFolder}
               loading={loadingFolders}
             />
           </div>
-        </div>
+        </ResizablePanel>
 
-        {/* Mail list */}
-        <div className="w-80 border-r border-secondary-200 overflow-y-auto bg-white" data-testid="mail-list-pane">
+        {/* Mail list — resizable */}
+        <ResizablePanel
+          initialWidth={320}
+          minWidth={200}
+          maxWidth={500}
+          className="border-r border-secondary-200 overflow-y-auto bg-white"
+          data-testid="mail-list-pane"
+        >
           <MailList
             mails={mails}
             selectedMailId={selectedMail?.id || null}
@@ -432,10 +490,14 @@ export function MailPage() {
             pageSize={PAGE_SIZE}
             onPageChange={setMailsPage}
           />
-        </div>
+        </ResizablePanel>
 
-        {/* Reading pane */}
-        <div className="flex-1 overflow-y-auto bg-white" data-testid="mail-detail-pane">
+        {/* Reading pane — flex-1, not resizable */}
+        <ResizablePanel
+          resizable={false}
+          className="overflow-y-auto bg-white"
+          data-testid="mail-detail-pane"
+        >
           <MailDetail
             mail={selectedMail}
             loading={loadingMail}
@@ -446,7 +508,7 @@ export function MailPage() {
             onDownloadAttachment={handleDownloadAttachment}
             downloadingAttachmentId={downloadingAttachmentId}
           />
-        </div>
+        </ResizablePanel>
       </div>
 
       <ComposeModal
@@ -461,4 +523,4 @@ export function MailPage() {
       />
     </div>
   );
-}
+}