Android Sudoku App Development

I need a complete, production-ready Sudoku game built for the Google Play Store, coded natively for Android (Kotlin is preferred, though I’m open to Flutter or React Native if you can guarantee equal performance). The look and feel must be unmistakably modern—clean lines, subtle animations, bold yet uncluttered colour choices—rather than the classic or ultra-minimal styles.

Core gameplay features
• Five difficulty tiers: Very Easy, Easy, Medium, Hard, Expert
• AdMob banners and interstitials integrated without disrupting play
• Win-loss based global leaderboard that refreshes in real time
• Points awarded after every solved grid (these points do not drive the leaderboard but will unlock badges or achievements later)
• Share button that generates social-media links so players can invite friends to download the app and compare standings
• Friend-vs-friend ranking comparison inside the app
• Millisecond-accurate in-game timer
• Fully offline playable grids with cloud sync when a connection returns
• Lightweight, responsive UI with buttery-smooth grid interactions

Nice-to-have additions (quote separately if they materially change cost)
• Daily challenge puzzle that expires at midnight local time
• Dark mode that switches automatically with system settings
• Three-level hint system with configurable penalties
• Toggleable sound effects and subtle vibration feedback

Deliverables
1. Source code with clear comments and a README covering build steps, tech stack, and third-party libraries
2. Two signed APK/AAB builds (debug and release) ready for Play Console upload
3. AdMob test IDs replaced with my live IDs in the final release build
4. Project doc listing all APIs, database schema (if any), and leaderboard back-end setup
5. Post-launch bug-fix support for an agreed period

Acceptance criteria
• The game passes Play Store review on the first submission
• Cold start to first interaction under three seconds on a mid-range device
• No memory leaks detected by Android Profiler after a 30-minute stress test
• Leaderboard updates propagate across two devices in under five seconds on a 4G connection

Please outline:
• Total fixed price broken into milestones (design, core mechanics, leaderboard backend, polishing/testing, Play Store release)
• Calendar-based timeline for each milestone
• Technology stack you intend to use—including back-end service for leaderboard (Firebase Realtime DB, Firestore, custom server, etc.)

I’m ready to move quickly once I review your proposed stack, timeline, and cost.