refactor: reduce duplication and improve readability of settings page

The settings page ([lib/app/modules/settings/views/](https://github.com/CCExtractor/taskwarrior-flutter/tree/main/lib/app/modules/settings/views)) has significant code duplication and readability
issues:

- Repeated section boilerplate - _buildSectionHeader() and _buildSettingsCard() are called manually for
every settings group with the same styling logic.
- Near-identical toggle widgets - 4 separate files (settings_page_on_task_start_list_tile_trailing.dart,
settings_page_on_task_create_list_tile_trailing.dart, settings_page_highlist_task_list_tile_trailing.dart,
settings_page_enable_24hr_format_list_tile_trailing.dart) each containing ~30 lines that all do the same
thing: Obx(() => Switch(...)).
- Hardcoded language names - _getLanguageName() duplicates what should be part of the SupportedLanguage enum, requiring updates in multiple places when a language is added.
- Large inline widgets - The delete-tasks confirmation dialog (~80 lines) is inlined directly in the body,
hurting readability.

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Proposed Changes

1. Introduce SettingsGroup widget + SettingsItemConfig sealed class
A single reusable widget that accepts title, icon, and a List and handles section
header, card styling, and dividers internally. Three item types:
- SettingsToggleItem - title, subtitle, RxBool value, optional prefsKey (auto-saves to SharedPreferences),
optional onChanged for extra side effects
- SettingsDropdownItem - title, subtitle, Rx value, options list, labelBuilder, onChanged
- SettingsCustomItem wraps any custom widget

2. Add nativeName getter to SupportedLanguage
Centralizes display names so Dart's exhaustive switch enforces updates when new languages are added.

3. Extract delete-tasks tile into SettingsPageDeleteTasksTile
Moves the FutureBuilder + confirmation dialog into its own widget file.

4. Delete absorbed files
Remove the 5 single-purpose trailing widget files whose logic is now handled by SettingsGroup.

Start in lib/app/modules/settings/views/ and read the settings page plus the four listed toggle trailing files. Trace SupportedLanguage and the inline delete-tasks confirmation before planning the shared SettingsGroup, language-name getter, and extracted delete-tasks tile. Done means repeated section and toggle logic is consolidated, language names have one source, the dialog is extracted, and the five absorbed files are removed.

索引モデルが issue の本文から書いたものです。