Switched to Material 3 Theming and fixed light mode issues

[TheGuyDangerous]

This pull request includes various updates to improve the issue templates, refactor the Flutter app code, and enhance the theme handling across multiple screens. The most important changes include the addition of new issue templates, refactoring the main app structure, and updating the theme integration.

Issue Template Updates:

• .github/workflows/ISSUE_TEMPLATE/custom.md: Added a new custom issue template.
• .github/workflows/ISSUE_TEMPLATE/feature_request.md: Added a new feature request template.

Flutter App Refactoring:

• lib/main.dart: Refactored the main app structure to use a SplashScreen and updated the theme handling to use ThemeProvider. [1] [2]
• lib/screens/home/home_screen.dart: Replaced BottomNavigationBar with NavigationBar and removed ThemeProvider usage. [1] [2]
• lib/screens/library/library_screen_state.dart: Updated the screen to use the theme from Theme.of(context) and removed ThemeProvider usage. [1] [2] [3] [4] [5]
• lib/screens/license/license_screen.dart: Updated the screen to use the theme from Theme.of(context) and removed ThemeProvider usage. [1] [2]
• lib/screens/onboarding_screen.dart: Removed AppTheme usage and updated the onboarding screen to use the theme from Theme.of(context). [1] [2] [3] [4] [5]

Other Changes:

• .github/workflows/build_and_release.yml: Updated the path for caching Pub dependencies.
• README.md: Changed the "Download" section header to a sub-header.