refactor: reduce CalDAV sync to login and manual sync button only

- Remove auto-login sync in AuthGuard
- Remove 10s interval sync and syncAndReload in calendar tab
- Remove lazy syncOnce pattern in ChatService AI callbacks
- Remove CaldavService dependency from ChatService constructor

apps/client/src/components/AuthGuard.tsx

@@ -62,11 +62,6 @@ export const AuthGuard = ({ children }: AuthGuardProps) => {
       if (!useAuthStore.getState().isAuthenticated) return;
       await preloadAppData();
       setDataReady(true);
-      try {
-        await CaldavConfigService.sync();
-      } catch {
-        // No CalDAV config or sync failed — not critical
-      }
     };
     init();
   }, [loadStoredUser]);