refactor(settings): 统一语言设置，合并 interface_language 和 ai_language

- 后端 Schema 将 interface_language 和 ai_language 合并为 language
- 前端设置界面只保留一个语言选项
- AI 回复语言统一使用 language 设置
- 更新协议文档
- 新增数据库迁移脚本

apps/lib/l10n/app_localizations_en.dart

@@ -226,10 +226,7 @@ class AppLocalizationsEn extends AppLocalizations {
   String get settingsAccountAndDataTitle => 'Account Data';
 
   @override
-  String get settingsInterfaceLanguage => 'Interface Language';
-
-  @override
-  String get settingsAiLanguage => 'AI Response Language';
+  String get settingsLanguage => 'Language';
 
   @override
   String get settingsNotificationAllow => 'Allow Notifications';
@@ -274,10 +271,6 @@ class AppLocalizationsEn extends AppLocalizations {
   String get settingsCoinHeroSubtitle =>
       'Credits will be used for casting and related services later.';
 
-  @override
-  String get settingsAiLanguageHint =>
-      'This field will align with profiles.settings.preferences.ai_language once the real preference flow is connected.';
-
   @override
   String get settingsTimezone => 'Time Zone';
 
@@ -285,13 +278,6 @@ class AppLocalizationsEn extends AppLocalizations {
   String get settingsTimezoneHint =>
       'This field will align with profiles.settings.preferences.timezone and later provide a real time zone picker.';
 
-  @override
-  String get settingsCountry => 'Country/Region';
-
-  @override
-  String get settingsCountryHint =>
-      'This field will align with profiles.settings.preferences.country and later provide a region picker.';
-
   @override
   String get settingsPrivacyProfileVisibility => 'Profile Visibility';
 
@@ -484,6 +470,9 @@ class AppLocalizationsEn extends AppLocalizations {
   @override
   String get settingsCoinPackPopularBadge => 'Popular';
 
+  @override
+  String get settingsCoinPackNewUserBadge => 'Only Once';
+
   @override
   String get settingsPurchaseButton => 'Pay Now';
 
@@ -1302,4 +1291,66 @@ class AppLocalizationsEn extends AppLocalizations {
 
   @override
   String get paymentPending => 'Apple is processing, please wait';
+
+  @override
+  String get notifyCenterTitle => 'Notifications';
+
+  @override
+  String get notifyMarkAllRead => 'Mark all read';
+
+  @override
+  String get notifyLoadFailed => 'Failed to load';
+
+  @override
+  String get notifyRetry => 'Retry';
+
+  @override
+  String get notifyEmpty => 'No notifications';
+
+  @override
+  String get timeJustNow => 'Just now';
+
+  @override
+  String timeMinutesAgo(int minutes) {
+    return '${minutes}m ago';
+  }
+
+  @override
+  String timeHoursAgo(int hours) {
+    return '${hours}h ago';
+  }
+
+  @override
+  String timeDaysAgo(int days) {
+    return '${days}d ago';
+  }
+
+  @override
+  String get pointsLedgerTitle => 'Points Ledger';
+
+  @override
+  String get pointsLedgerEmpty => 'No records yet';
+
+  @override
+  String get pointsLedgerTypeRegister => 'Registration bonus';
+
+  @override
+  String get pointsLedgerTypePurchase => 'Purchase credits';
+
+  @override
+  String get pointsLedgerTypeConsume => 'AI chat cost';
+
+  @override
+  String get pointsLedgerTypeAdjust => 'System adjustment';
+
+  @override
+  String get pointsLedgerTypeRefund => 'Refund';
+
+  @override
+  String pointsLedgerBalance(int balance) {
+    return 'Balance $balance';
+  }
+
+  @override
+  String get retry => 'Retry';
 }