feat: plugin ribbon toolbar + mail response fixes

- Add pluginToolbarStore for plugin toolbar item registration
- Add PluginToolbar component (slim ribbon bar under TopBar)
- Update AppShell to render PluginToolbar between TopBar and content
- Mail page registers toolbar items (account select, search, compose, sync, settings)
- Remove old mail top bar, use plugin toolbar instead
- Fix mail response: arrays for addresses, add from_name/body_html/sanitized_html/date

frontend/src/pages/Mail.tsx

@@ -15,6 +15,7 @@ import { MailDetail } from '@/components/mail/MailDetail';
 import { ComposeModal, type ComposeMode } from '@/components/mail/ComposeModal';
 import { SharedMailboxSelector } from '@/components/mail/SharedMailboxSelector';
 import { MailSearchBar } from '@/components/mail/MailSearchBar';
+import { usePluginToolbarStore } from '@/store/pluginToolbarStore';
 import {
   fetchAccounts,
   fetchFolders,
@@ -280,6 +281,57 @@ export function MailPage() {
     setSearchQuery('');
   }, []);
 
+  // Register toolbar items
+  const { registerItems, unregisterPlugin } = usePluginToolbarStore();
+  useEffect(() => {
+    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',
+        label: 'Suchen',
+        type: 'search',
+        group: 'search',
+        searchPlaceholder: 'E-Mails durchsuchen...',
+        onSearch: handleSearch,
+      },
+      {
+        id: 'compose',
+        plugin: 'mail',
+        label: 'Verfassen',
+        group: 'actions',
+        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,
+      },
+      {
+        id: 'sync',
+        plugin: 'mail',
+        label: 'Sync',
+        group: 'actions',
+        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 */ },
+      },
+      {
+        id: 'settings',
+        plugin: 'mail',
+        label: 'Einstellungen',
+        group: 'actions',
+        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',
+      },
+    ]);
+    return () => unregisterPlugin('mail');
+  }, [registerItems, unregisterPlugin, accounts, selectedAccountId, handleAccountSwitch, handleSearch, handleCompose]);
+
   if (loadingAccounts) {
     return (
       <div className="flex items-center justify-center py-12" data-testid="mail-page-loading">
@@ -306,29 +358,6 @@ export function MailPage() {
 
   return (
     <div className="flex flex-col h-full" data-testid="mail-page">
-      {/* Top bar */}
-      <div className="flex items-center gap-3 px-4 py-3 border-b border-secondary-200 bg-white">
-        <SharedMailboxSelector
-          accounts={accounts}
-          selectedAccountId={selectedAccountId}
-          onSelect={handleAccountSwitch}
-        />
-        <div className="flex-1 max-w-md">
-          <MailSearchBar onSearch={handleSearch} />
-        </div>
-        <div className="flex items-center gap-2">
-          <Button size="sm" onClick={handleCompose} data-testid="compose-btn">
-            <svg className="w-4 h-4 mr-1" fill="none" viewBox="0 0 24 24" stroke="currentColor" aria-hidden="true">
-              <path strokeLinecap="round" strokeLinejoin="round" strokeWidth={2} d="M12 4v16m8-8H4" />
-            </svg>
-            {t('mail.compose')}
-          </Button>
-          <Link to="/mail/settings" className="text-sm text-primary-600 hover:text-primary-700">
-            {t('mail.settings')}
-          </Link>
-        </div>
-      </div>
-
       {error && (
         <div className="mb-2 p-3 bg-danger-50 border border-danger-200 rounded-md" role="alert" data-testid="mail-error">
           <p className="text-sm text-danger-700">{error}</p>