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/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"
   }
 }