refactor: Refactor null safety and improve code quality across multiple files
- Updated notifications_page.dart to handle potential null values for author avatar and display name.
- Enhanced profile_page.dart to ensure safe access to profile description and gallery items.
- Modified gallery_action_buttons.dart to utilize Riverpod for managing favorite state.
- Changed gallery_photo_view.dart to safely check for photo alt text.
- Improved gallery_preview.dart to filter photos based on non-null thumbnails.
- Updated justified_gallery_view.dart to handle aspect ratios safely and ensure thumbnails are not null.
- Refactored timeline_item.dart to use Riverpod for gallery data and ensure safe access to actor details.
- Updated pubspec.yaml and pubspec.lock to include new dependencies and overrides for analyzer and linting packages.