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)

app/plugins/builtins/mail/routes.py

@@ -73,6 +73,8 @@ from app.plugins.builtins.mail.services import (
     folder_to_response,
     forward_mail,
     get_account_password,
+    imap_create_folder,
+    imap_delete_folder,
     imap_delete_mail,
     imap_move_mail,
     imap_sync_account,
@@ -514,6 +516,14 @@ async def create_folder(
     )
     db.add(folder)
     await db.flush()
+    try:
+        await imap_create_folder(db, acc_id, data.imap_name, tenant_id)
+    except Exception:
+        import logging as _logging
+
+        _logging.getLogger(__name__).warning(
+            "create_folder: IMAP CREATE failed (non-critical) for %s", data.imap_name
+        )
     return folder_to_response(folder)
 
 
@@ -559,6 +569,14 @@ async def delete_folder(
         raise HTTPException(404, detail={"detail": "Folder not found", "code": "not_found"})
     account = await _get_account(db, folder.account_id, tenant_id, user_id)
     await _check_delegate_full_access(db, account, user_id)
+    try:
+        await imap_delete_folder(db, f_id, tenant_id)
+    except Exception:
+        import logging as _logging
+
+        _logging.getLogger(__name__).warning(
+            "delete_folder: IMAP DELETE failed (non-critical) for folder %s", f_id
+        )
     await db.delete(folder)