chore(i18n): 移除繁体中文支持

- 删除 lib/l10n/translations/zh_tw.dart
- app_localizations.dart：去掉 zh_TW import、_localizedValues 条目、
  isSupported 不再 fallback Hant；supportedLocales 只保留 en + zh；
  localeDisplayNames / getLocaleKey / parseLocaleKey 全部简化
- settings/widgets/language_section.dart：allLanguages 去掉 zh_TW 选项
- utils/locale_utils.dart：注释里去掉 zh_TW

繁体中文翻译文件 zh_tw.dart 内容不全且过时（很多 key 用 fallback en），
继续维护性价比低；现在 zh.dart 已经够用，需要时未来还可恢复。

Author: shiyi-g

lib/l10n/app_localizations.dart

@@ -2,7 +2,6 @@ import 'package:flutter/material.dart';
 
 import 'translations/en.dart' as t_en;
 import 'translations/zh.dart' as t_zh;
-import 'translations/zh_tw.dart' as t_zh_tw;
 
 /// App translations for all supported languages
 class AppLocalizations {
@@ -19,14 +18,10 @@ class AppLocalizations {
   static final Map<String, Map<String, String>> _localizedValues = {
     'en': t_en.en,
     'zh': t_zh.zh,
-    'zh_TW': t_zh_tw.zhTW,
   };
 
   String get(String key) {
-    String langCode = locale.languageCode;
-    if (locale.scriptCode == 'Hant' || locale.countryCode == 'TW' || locale.countryCode == 'HK') {
-      langCode = 'zh_TW';
-    }
+    final langCode = locale.languageCode;
     return _localizedValues[langCode]?[key] ?? _localizedValues['en']?[key] ?? key;
   }
 
@@ -65,31 +60,20 @@ class _AppLocalizationsDelegate extends LocalizationsDelegate<AppLocalizations>
 const supportedLocales = [
   Locale('en'),
   Locale('zh'),
-  Locale.fromSubtags(languageCode: 'zh', scriptCode: 'Hant'),
 ];
 
-/// Language display names (legacy, use allLanguages in settings_screen.dart)
+/// Language display names
 const localeDisplayNames = {
   'en': 'English',
   'zh': '简体中文',
-  'zh_TW': '繁體中文',
 };
 
 /// Get locale key for storage
 String getLocaleKey(Locale locale) {
-  if (locale.languageCode == 'zh') {
-    if (locale.scriptCode == 'Hant' || locale.countryCode == 'TW' || locale.countryCode == 'HK') {
-      return 'zh_TW';
-    }
-    return 'zh';
-  }
   return locale.languageCode;
 }
 
 /// Parse locale from key
 Locale parseLocaleKey(String key) {
-  if (key == 'zh_TW') {
-    return const Locale.fromSubtags(languageCode: 'zh', scriptCode: 'Hant');
-  }
   return Locale(key);
 }

lib/l10n/translations/zh_tw.dart

@@ -14,11 +14,6 @@ class LanguageSection extends ConsumerWidget {
   static const allLanguages = {
     'en': {'flag': '🇺🇸', 'native': 'English', 'english': 'English'},
     'zh': {'flag': '🇨🇳', 'native': '简体中文', 'english': 'Simplified Chinese'},
-    'zh_TW': {
-      'flag': '🇹🇼',
-      'native': '繁體中文',
-      'english': 'Traditional Chinese'
-    },
   };
 
   @override

lib/screens/settings/widgets/language_section.dart

@@ -5,7 +5,7 @@ bool isZhLocale(BuildContext context) {
   return Localizations.localeOf(context).languageCode == 'zh';
 }
 
-/// Get locale key for comparison (e.g., 'en', 'zh', 'zh_TW')
+/// Get locale key for comparison (e.g., 'en', 'zh')
 String? getLocaleKey(Locale? locale) {
   if (locale == null) return null;
   if (locale.countryCode != null && locale.countryCode!.isNotEmpty) {