refactor: reduce duplication and improve readability of settings page

lib/app/modules/settings/views/ から始め、設定ページと、一覧にある4つのtoggle trailingファイルを読みます。共有のSettingsGroup、言語名getter、抽出したdelete-tasksタイルを計画する前に、SupportedLanguageとインラインのタスク削除確認をたどります。完了とは、繰り返されているセクションとtoggleのロジックが統合され、言語名のソースが1つになり、ダイアログが抽出され、取り込まれた5つのファイルが削除されている状態です。

The settings page (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.

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.