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/components/mail/MailFolderTree.tsx

@@ -1,22 +1,79 @@
 /**
  * Mail folder tree sidebar component.
- * Shows hierarchical folder list with unread badges.
+ * Shows accounts as top-level nodes with their folders nested underneath.
+ * Folders come as a flat list with parent_id and are built into a tree.
  */
 
-import React from 'react';
+import React, { useState, useMemo } from 'react';
 import clsx from 'clsx';
 import { useTranslation } from 'react-i18next';
-import type { MailFolder } from '@/api/mail';
+import type { MailAccount, MailFolder } from '@/api/mail';
 import { Badge } from '@/components/ui/Badge';
 import { EmptyState } from '@/components/ui/EmptyState';
 
 export interface MailFolderTreeProps {
+  accounts: MailAccount[];
   folders: MailFolder[];
   selectedFolderId: string | null;
   onSelect: (folderId: string) => void;
   loading: boolean;
 }
 
+/**
+ * Build a hierarchical tree from a flat folder list for a given account.
+ */
+function buildFolderTree(folders: MailFolder[], accountId: string): MailFolder[] {
+  const accountFolders = folders.filter((f) => f.account_id === accountId);
+  const byId = new Map<string, MailFolder & { children: MailFolder[] }>();
+
+  for (const f of accountFolders) {
+    byId.set(f.id, { ...f, children: [] });
+  }
+
+  const roots: MailFolder[] = [];
+
+  for (const f of accountFolders) {
+    const node = byId.get(f.id)!;
+    if (f.parent_id && byId.has(f.parent_id)) {
+      byId.get(f.parent_id)!.children.push(node);
+    } else {
+      roots.push(node);
+    }
+  }
+
+  return roots;
+}
+
+function ChevronIcon({ expanded }: { expanded: boolean }) {
+  return (
+    <svg
+      className={clsx('w-3 h-3 transition-transform flex-shrink-0', expanded ? 'rotate-90' : '')}
+      fill="none"
+      viewBox="0 0 24 24"
+      stroke="currentColor"
+      aria-hidden="true"
+    >
+      <path strokeLinecap="round" strokeLinejoin="round" strokeWidth={2} d="M9 5l7 7-7 7" />
+    </svg>
+  );
+}
+
+function FolderIcon() {
+  return (
+    <svg className="w-4 h-4 flex-shrink-0" fill="none" viewBox="0 0 24 24" stroke="currentColor" aria-hidden="true">
+      <path strokeLinecap="round" strokeLinejoin="round" strokeWidth={2} d="M3 7v10a2 2 0 002 2h14a2 2 0 002-2V9a2 2 0 00-2-2h-6l-2-2H5a2 2 0 00-2 2z" />
+    </svg>
+  );
+}
+
+function MailIcon() {
+  return (
+    <svg className="w-4 h-4 flex-shrink-0" fill="none" viewBox="0 0 24 24" stroke="currentColor" aria-hidden="true">
+      <path strokeLinecap="round" strokeLinejoin="round" strokeWidth={2} d="M3 8l7-7 7 7M5 19h10a2 2 0 002-2V8l-5-5H5a2 2 0 00-2 2v12a2 2 0 002 2z" />
+    </svg>
+  );
+}
+
 function FolderNode({
   folder,
   selectedFolderId,
@@ -28,36 +85,51 @@ function FolderNode({
   onSelect: (folderId: string) => void;
   depth: number;
 }) {
- const { t } = useTranslation();
+  const [expanded, setExpanded] = useState(true);
   const isSelected = folder.id === selectedFolderId;
   const hasChildren = folder.children && folder.children.length > 0;
 
   return (
     <div role="treeitem" aria-selected={isSelected} aria-label={folder.name}>
-      <button
-        onClick={() => onSelect(folder.id)}
-        className={clsx(
-          'w-full flex items-center gap-2 px-2 py-1.5 rounded-md text-sm min-h-touch',
-          'motion-safe:transition-colors focus:outline-none focus-visible:ring-2 focus-visible:ring-primary-500',
-          isSelected
-            ? 'bg-primary-50 text-primary-700 font-medium'
-            : 'hover:bg-secondary-50 text-secondary-700',
+      <div className="flex items-center" style={{ paddingLeft: `${depth * 12}px` }}>
+        {hasChildren ? (
+          <button
+            onClick={(e) => {
+              e.stopPropagation();
+              setExpanded(!expanded);
+            }}
+            className="flex-shrink-0 p-0.5 hover:bg-secondary-100 rounded"
+            aria-label={expanded ? 'Collapse' : 'Expand'}
+            data-testid={`folder-chevron-${folder.id}`}
+          >
+            <ChevronIcon expanded={expanded} />
+          </button>
+        ) : (
+          <span className="w-4 flex-shrink-0" />
         )}
-        data-testid={`folder-${folder.id}`}
-      >
-        <svg className="w-4 h-4 flex-shrink-0" fill="none" viewBox="0 0 24 24" stroke="currentColor" aria-hidden="true">
-          <path strokeLinecap="round" strokeLinejoin="round" strokeWidth={2} d="M3 7v10a2 2 0 002 2h14a2 2 0 002-2V9a2 2 0 00-2-2h-6l-2-2H5a2 2 0 00-2 2z" />
-        </svg>
-        <span className="flex-1 truncate text-left">{folder.name}</span>
-        {folder.unread_count > 0 && (
-          <Badge variant="primary" className="text-xs">{folder.unread_count}</Badge>
-        )}
-        {folder.total_count > 0 && folder.unread_count === 0 && (
-          <span className="text-xs text-secondary-400">{folder.total_count}</span>
-        )}
-      </button>
-      {hasChildren && (
-        <div className="ml-4" role="group">
+        <button
+          onClick={() => onSelect(folder.id)}
+          className={clsx(
+            'flex-1 flex items-center gap-2 px-2 py-1.5 rounded-md text-sm min-h-touch',
+            'motion-safe:transition-colors focus:outline-none focus-visible:ring-2 focus-visible:ring-primary-500',
+            isSelected
+              ? 'bg-primary-50 text-primary-700 font-medium'
+              : 'hover:bg-secondary-50 text-secondary-700',
+          )}
+          data-testid={`folder-${folder.id}`}
+        >
+          <FolderIcon />
+          <span className="flex-1 truncate text-left">{folder.name}</span>
+          {folder.unread_count > 0 && (
+            <Badge variant="primary" className="text-xs">{folder.unread_count}</Badge>
+          )}
+          {folder.total_count > 0 && folder.unread_count === 0 && (
+            <span className="text-xs text-secondary-400">{folder.total_count}</span>
+          )}
+        </button>
+      </div>
+      {hasChildren && expanded && (
+        <div role="group">
           {folder.children!.map((child) => (
             <FolderNode
               key={child.id}
@@ -73,7 +145,52 @@ function FolderNode({
   );
 }
 
-export function MailFolderTree({ folders, selectedFolderId, onSelect, loading }: MailFolderTreeProps) {
+function AccountNode({
+  account,
+  folders,
+  selectedFolderId,
+  onSelect,
+}: {
+  account: MailAccount;
+  folders: MailFolder[];
+  selectedFolderId: string | null;
+  onSelect: (folderId: string) => void;
+}) {
+  const [expanded, setExpanded] = useState(true);
+  const accountFolders = useMemo(
+    () => buildFolderTree(folders, account.id),
+    [folders, account.id],
+  );
+
+  return (
+    <div role="treeitem" aria-label={account.email}>
+      <button
+        onClick={() => setExpanded(!expanded)}
+        className="w-full flex items-center gap-1 px-1 py-1.5 rounded-md text-sm font-semibold text-secondary-900 hover:bg-secondary-50"
+        data-testid={`account-${account.id}`}
+      >
+        <ChevronIcon expanded={expanded} />
+        <MailIcon />
+        <span className="flex-1 truncate text-left">{account.email}</span>
+      </button>
+      {expanded && (
+        <div role="group" className="ml-2">
+          {accountFolders.map((folder) => (
+            <FolderNode
+              key={folder.id}
+              folder={folder}
+              selectedFolderId={selectedFolderId}
+              onSelect={onSelect}
+              depth={0}
+            />
+          ))}
+        </div>
+      )}
+    </div>
+  );
+}
+
+export function MailFolderTree({ accounts, folders, selectedFolderId, onSelect, loading }: MailFolderTreeProps) {
   const { t } = useTranslation();
 
   if (loading) {
@@ -88,7 +205,7 @@ export function MailFolderTree({ folders, selectedFolderId, onSelect, loading }:
     );
   }
 
-  if (folders.length === 0) {
+  if (accounts.length === 0) {
     return (
       <EmptyState
         title={t('mail.noFolders')}
@@ -100,13 +217,13 @@ export function MailFolderTree({ folders, selectedFolderId, onSelect, loading }:
 
   return (
     <div className="space-y-1" role="tree" data-testid="folder-tree-list">
-      {folders.map((folder) => (
-        <FolderNode
-          key={folder.id}
-          folder={folder}
+      {accounts.map((account) => (
+        <AccountNode
+          key={account.id}
+          account={account}
+          folders={folders}
           selectedFolderId={selectedFolderId}
           onSelect={onSelect}
-          depth={0}
         />
       ))}
     </div>