[FIX] Fixes #264: onboarding overflow + Flutter 3/Dart 3 compatibility

[ObaidAbdullah16]

Fixes #264

Summary

This PR fixes the onboarding RenderFlex overflow and updates Flood Mobile to build/run on modern Flutter (Flutter 3.x / Dart 3 + Android Gradle Plugin 8) by migrating deprecated APIs, updating dependencies, and adjusting a few widgets/tests for compatibility.

Onboarding overflow fix

• Removed the hard-coded height: 250 constraint in
  lib/Pages/onboarding_main_screen/widgets/onboard_page.dart
  and made the text area flexible/scrollable to prevent RenderFlex overflow on small screens and large font scaling.

Flutter 3 / Dart 3 / AGP 8 compatibility

• Updated/replaced incompatible dependencies and migrated deprecated/breaking APIs across the app to compile on modern Flutter.
• RSS screen: replaced DropdownButtonFormField2 usage with Flutter’s built-in DropdownButtonFormField to avoid breaking API differences.
• Updated l10n generation:
  • Added l10n.yaml
  • Updated l10n wiring/imports to align with flutter gen-l10n
• Android: enabled Android 13+ back callback via android:enableOnBackInvokedCallback="true" in the manifest.
• Updated platform plugin registrants (Linux/macOS/Windows) as a result of plugin/dependency changes.
• Updated .gitignore to avoid committing generated/build artifacts.

Tests

• Fixed unit tests using SharedPreferences by initializing the test binding and using SharedPreferences.setMockInitialValues.
• Improved widget test reliability (ensure widgets are visible before tapping where needed).
• Removed the default Flutter template counter test (test/widget_test.dart) which no longer matches the app.

Verification

• flutter gen-l10n
• flutter analyze
• flutter test
• flutter run -d emulator-5554