feat(frontend): add multi-language support (en, zh-Hans, zh-Hant, ja)

frontend/lib/providers/settings_provider.dart

@@ -13,11 +13,13 @@ class SettingsProvider with ChangeNotifier {
   Color _themeColor = Colors.blue;
   ThemeMode _themeMode = ThemeMode.system;
   bool _livePreviewThumbnailsEnabled = false;
+  Locale? _locale;
 
   String get baseUrl => _baseUrl;
   Color get themeColor => _themeColor;
   ThemeMode get themeMode => _themeMode;
   bool get livePreviewThumbnailsEnabled => _livePreviewThumbnailsEnabled;
+  Locale? get locale => _locale;
 
   SettingsProvider() {
     _loadSettings();
@@ -36,6 +38,41 @@ class SettingsProvider with ChangeNotifier {
     }
     _livePreviewThumbnailsEnabled =
         prefs.getBool('livePreviewThumbnailsEnabled') ?? false;
+    
+    final languageCode = prefs.getString('languageCode');
+    final scriptCode = prefs.getString('scriptCode');
+    final countryCode = prefs.getString('countryCode');
+    if (languageCode != null) {
+      _locale = Locale.fromSubtags(
+        languageCode: languageCode,
+        scriptCode: scriptCode,
+        countryCode: countryCode,
+      );
+    }
+    
+    notifyListeners();
+  }
+
+  void setLocale(Locale? newLocale) async {
+    _locale = newLocale;
+    final prefs = await SharedPreferences.getInstance();
+    if (newLocale == null) {
+      await prefs.remove('languageCode');
+      await prefs.remove('scriptCode');
+      await prefs.remove('countryCode');
+    } else {
+      await prefs.setString('languageCode', newLocale.languageCode);
+      if (newLocale.scriptCode != null) {
+        await prefs.setString('scriptCode', newLocale.scriptCode!);
+      } else {
+        await prefs.remove('scriptCode');
+      }
+      if (newLocale.countryCode != null) {
+        await prefs.setString('countryCode', newLocale.countryCode!);
+      } else {
+        await prefs.remove('countryCode');
+      }
+    }
     notifyListeners();
   }