Mobile app development - Duolingo clone

I need a mobile application built from the ground up that runs natively on both iOS and Android. Its sole focus is education, so everything you deliver should make learning clear, engaging, and trackable for students.

The core build should include interactive lesson pages, short quizzes, progress tracking, and push-notification reminders. A single code-base framework - Flutter + a dedicated web app with a database. Project is already half baked, 60% ready, and can be picked up.

Key milestones
• Wireframes and UI/UX signed off
• First functional prototype on TestFlight and Google Play Internal Testing
• Final production build, store submission, and hand-over of all source files plus concise setup documentation

Acceptance criteria
• Clean, well-commented code that compiles without warnings on the latest iOS and Android versions
• UI matches the approved designs across common device sizes
• Core content available offline with graceful data sync when a connection returns
• Basic analytics and crash reporting enabled
• All designated test cases pass before final approval

Include in your reply a brief outline of your proposed architecture, any third-party libraries you expect to use, and a realistic timeline so we can move forward quickly.