feat: add runtime i18n with English, Vietnamese, and Simplified Chinese support

- Implement runtime i18n using MutationObserver for automatic DOM translation
- Add language switcher dropdown in dashboard header (EN/VI/ZH)
- Support 3 languages: English (default), Tiếng Việt, 简体中文
- Add translation files: vi.json (197 entries), zh-CN.json (513 entries, cleaned)
- Translate dashboard UI: sidebar menu, header, settings, MITM page
- Use cookie-based locale persistence with /api/locale endpoint
- Zero component changes required - translations applied at runtime
- Fix Header flicker on route change with key={pathname}

Co-authored-by: eachann <each1024@qq.com>
Based on PR #247 from decolua/9router with runtime approach

Made-with: Cursor

src/shared/components/index.js

@@ -17,6 +17,7 @@ export { default as OAuthModal } from "./OAuthModal";
 export { default as ModelSelectModal } from "./ModelSelectModal";
 export { default as ManualConfigModal } from "./ManualConfigModal";
 export { default as UsageStats } from "./UsageStats";
+export { default as LanguageSwitcher } from "./LanguageSwitcher";
 export { default as RequestLogger } from "./RequestLogger";
 export { default as KiroAuthModal } from "./KiroAuthModal";
 export { default as KiroOAuthWrapper } from "./KiroOAuthWrapper";