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/features/settings/presentation/screens/general_settings_screen.dart

@@ -54,40 +54,20 @@ class _GeneralSettingsScreenState extends State<GeneralSettingsScreen> {
             children: [
               SettingsMenuTile(
                 icon: Icons.language_rounded,
-                title: l10n.settingsInterfaceLanguage,
+                title: l10n.settingsLanguage,
                 subtitle: displayLanguageLabel(
                   l10n,
-                  _settings.preferences.interfaceLanguage,
-                ),
-                tint: colors.primary,
-                background: colors.surfaceContainerHighest,
-                onTap: () => _selectLanguage(
-                  _settings.preferences.interfaceLanguage,
-                  (lang) => setState(() {
-                    _settings = _settings.copyWith(
-                      preferences: _settings.preferences.copyWith(
-                        interfaceLanguage: lang,
-                      ),
-                    );
-                  }),
-                ),
-              ),
-              SettingsMenuTile(
-                icon: Icons.smart_toy_rounded,
-                title: l10n.settingsAiLanguage,
-                subtitle: displayLanguageLabel(
-                  l10n,
-                  _settings.preferences.aiLanguage,
+                  _settings.preferences.language,
                 ),
                 tint: colors.primary,
                 background: colors.surfaceContainerHighest,
                 showDivider: false,
                 onTap: () => _selectLanguage(
-                  _settings.preferences.aiLanguage,
+                  _settings.preferences.language,
                   (lang) => setState(() {
                     _settings = _settings.copyWith(
                       preferences: _settings.preferences.copyWith(
-                        aiLanguage: lang,
+                        language: lang,
                       ),
                     );
                   }),