feat: implement offline-first functionality with local storage

- Add IndexedDB storage layer for notes (src/db/localDB.ts)
- Implement sync manager with queue and conflict resolution (src/services/syncManager.ts)
- Add online/offline detection hook (src/hooks/useOnlineStatus.ts)
- Load notes from local storage immediately on app startup
- Add sync status UI indicators (offline badge, pending count)
- Auto-sync every 5 minutes when online
- Queue operations when offline, sync when connection restored
- Fix note content update when synced from server while viewing
- Retry failed sync operations up to 5 times
- Temporary IDs for offline-created notes

src/hooks/useOnlineStatus.ts

@@ -0,0 +1,20 @@
+import { useState, useEffect } from 'react';
+
+export function useOnlineStatus() {
+  const [isOnline, setIsOnline] = useState(navigator.onLine);
+
+  useEffect(() => {
+    const handleOnline = () => setIsOnline(true);
+    const handleOffline = () => setIsOnline(false);
+
+    window.addEventListener('online', handleOnline);
+    window.addEventListener('offline', handleOffline);
+
+    return () => {
+      window.removeEventListener('online', handleOnline);
+      window.removeEventListener('offline', handleOffline);
+    };
+  }, []);
+
+  return isOnline;
+}