feat: mail phase 4 - IMAP folder create/delete + auto-sync

- Add imap_create_folder() and imap_delete_folder() service functions
- Call IMAP CREATE/DELETE in folder create/delete endpoints
- Add auto_sync_all_accounts() background task (every 5 min)
- Start auto-sync loop on plugin activation via asyncio.create_task
- Frontend: working sync button with isSyncing state and toast feedback
- i18n: syncSuccess, syncFailed, syncing, autoSyncEnabled keys (de/en)

frontend/src/i18n/locales/de.json

@@ -559,6 +559,10 @@
     "sortFrom": "Absender",
     "sortSubject": "Betreff",
     "sortAsc": "Aufsteigend",
-    "sortDesc": "Absteigend"
+    "sortDesc": "Absteigend",
+    "syncSuccess": "Synchronisierung erfolgreich",
+    "syncFailed": "Synchronisierung fehlgeschlagen",
+    "syncing": "Synchronisiere...",
+    "autoSyncEnabled": "Auto-Sync aktiv"
   }
 }

frontend/src/i18n/locales/en.json

@@ -559,6 +559,10 @@
     "sortFrom": "From",
     "sortSubject": "Subject",
     "sortAsc": "Ascending",
-    "sortDesc": "Descending"
+    "sortDesc": "Descending",
+    "syncSuccess": "Sync successful",
+    "syncFailed": "Sync failed",
+    "syncing": "Syncing...",
+    "autoSyncEnabled": "Auto-sync enabled"
   }
 }

frontend/src/pages/Mail.tsx

@@ -36,6 +36,7 @@ import {
   moveMail,
   saveDraft,
   updateDraft,
+  triggerSync,
   type MailAccount,
   type MailFolder,
   type Mail,
@@ -80,6 +81,7 @@ export function MailPage() {
   const [showMoveDropdown, setShowMoveDropdown] = useState(false);
   const [sortBy, setSortBy] = useState<'date' | 'from' | 'subject'>('date');
   const [sortOrder, setSortOrder] = useState<'asc' | 'desc'>('desc');
+  const [isSyncing, setIsSyncing] = useState(false);
 
   // Load accounts
   const loadAccounts = useCallback(async () => {
@@ -585,14 +587,27 @@ export function MailPage() {
       {
         id: 'sync',
         plugin: 'mail',
-        label: 'Sync',
+        label: isSyncing ? t('mail.syncing') : 'Sync',
         group: 'tools',
+        disabled: isSyncing || !selectedAccountId,
         icon: (
-          <svg className="w-3.5 h-3.5" fill="none" viewBox="0 0 24 24" stroke="currentColor">
+          <svg className={`w-3.5 h-3.5 ${isSyncing ? 'animate-spin' : ''}`} 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 */ },
+        onClick: async () => {
+          if (!selectedAccountId) return;
+          setIsSyncing(true);
+          try {
+            await triggerSync(selectedAccountId);
+            await loadMails();
+            toast.success(t('mail.syncSuccess'));
+          } catch (err) {
+            toast.error(err instanceof Error ? err.message : String(err));
+          } finally {
+            setIsSyncing(false);
+          }
+        },
       },
     ];
     if (hasBulkSelection) {