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/app/(tabs)/calendar.tsx

@@ -17,7 +17,6 @@ import { Ionicons } from "@expo/vector-icons";
 import { useThemeStore } from "../../stores/ThemeStore";
 import BaseBackground from "../../components/BaseBackground";
 import { EventService } from "../../services";
-import { CaldavConfigService } from "../../services/CaldavConfigService";
 import { useEventsStore } from "../../stores";
 import { useDropdownPosition } from "../../hooks/useDropdownPosition";
 
@@ -108,25 +107,11 @@ const Calendar = () => {
     }
   }, [monthIndex, currentYear, setEvents]);
 
-  // Sync CalDAV in background, then reload events
-  const syncAndReload = useCallback(async () => {
-    try {
-      await CaldavConfigService.sync();
-      await loadEvents();
-    } catch {
-      // Sync failed — not critical
-    }
-  }, [loadEvents]);
-
-  // Load events instantly on focus, then sync in background periodically
+  // Load events from DB on focus
   useFocusEffect(
     useCallback(() => {
       loadEvents();
-      syncAndReload();
-
-      const interval = setInterval(syncAndReload, 10_000);
-      return () => clearInterval(interval);
-    }, [loadEvents, syncAndReload]),
+    }, [loadEvents]),
   );
 
   // Re-open overlay after back navigation from editEvent