Replaces the dialog pop-up for theme selection (Light, Dark, System)
with `SegmentedButton` widgets in the theme settings page.
The new UI uses:
- Light theme option on the left.
- System theme option in the middle.
- Dark theme option on the right.
- A checkmark icon (`Icons.check`) for the selected theme.
- Respective icons (`Icons.light_mode`, `Icons.auto_awesome`, `Icons.dark_mode`)
for unselected themes.
The `ModeSelectionDialog` has been removed as it's no longer necessary.
Note: Full UI and persistence testing could not be completed due to
sandbox limitations where the Flutter SDK was not available.
The core code implementation was verified.
* refactor: consolidate theme definitions by removing separate dark and light theme files
* feat: integrate dynamic color support and enhance theme settings management
* feat: add theme settings route and update theme management in app settings
* feat: enhance theme management by integrating high contrast support in various components
* feat: implement mode selection dialog for theme settings and enhance button functionality
* refactor: update theme import paths and consolidate theme provider files
* feat: enhance theme management by integrating theme selection based on audiobook playback
* refactor: update default value for useMaterialThemeFromSystem to false in theme settings
* refactor: adjust high contrast condition order in theme settings for consistency
* refactor: rename useMaterialThemeOfPlayingItem to useCurrentPlayerThemeThroughoutApp for clarity
* refactor: correct spelling in system theme provider and replace with updated implementation
* refactor: extract restore backup dialog into a separate widget for improved readability
* refactor: reorganize settings sections for clarity and improve restore dialog functionality
