MongoDB-backed version

What changed:
- Added an Express + MongoDB backend in server/src
- Sign up / sign in now use MongoDB instead of browser-only local storage
- User profile, task progress, and buddy check-in status are also synced to MongoDB
- Frontend keeps only the auth token + cached session/profile locally

Run locally:
1. Open a terminal in the project folder
2. Run: npm install
3. Run: npm run dev
4. Frontend: http://localhost:5173
5. Backend: http://localhost:4000

Important:
- The MongoDB connection string is already placed in .env
- For security, change JWT_SECRET before production use
- If you already added a Capacitor native project, after changing frontend code run:
  npm run cap:sync
