Refactor bootstrap and add file send support

[Dospacite]

Summary

• Refactors app startup into a Riverpod-based bootstrap flow with a dedicated app shell and theme module.
• Moves controller, transport, settings, and feature UI code into a clearer core/features layout.
• Adds Android file-send plumbing so Flutter can pass a payload file path to the foreground service.
• Updates the main UI wiring and widget test coverage to match the new app structure.

Testing

• Not run.
• Verify the app launches through AppBootstrap.load() and renders the home page with the overridden appControllerProvider.
• Verify sendFile from Flutter reaches MainActivity and the foreground service reads, deletes, and sends the payload file.
• Verify the existing connection and settings flows still load after the controller and transport file split.
• Verify test/app_widget_test.dart passes in the updated app structure.