feat: integrate language switcher into settings with modern segment control UI

- Move language switcher from header to settings modal for better organization
- Implement modern segment control UI instead of radio buttons for language selection
- Add language preference persistence in localStorage and backend settings
- Support instant language preview with cancel/revert functionality
- Remove standalone LanguageSwitcher component
- Improve initial language detection logic (localStorage -> browser -> default)
- Add proper i18n keys for language settings UI text

src/types.ts

@@ -30,4 +30,6 @@ export interface Settings {
   claudeConfigDir?: string;
   // 覆盖 Codex 配置目录（可选）
   codexConfigDir?: string;
+  // 首选语言（可选，默认中文）
+  language?: "en" | "zh";
 }