feat: add internationalization (i18n) to web dashboard — English + Chinese (#9453)

Add a lightweight i18n system to the web dashboard with English (default) and
Chinese language support. A language switcher with flag icons is placed in the
header bar, allowing users to toggle between languages. The choice persists
to localStorage.

Implementation:
- src/i18n/ — types, translation files (en.ts, zh.ts), React context + hook
- LanguageSwitcher component shows the *other* language's flag as the toggle
- I18nProvider wraps the app in main.tsx
- All 8 pages + OAuth components updated to use t() translation calls
- Zero new dependencies — pure React context + localStorage

web/src/main.tsx

@@ -1,10 +1,10 @@
 import { createRoot } from "react-dom/client";
-import { BrowserRouter } from "react-router-dom";
 import "./index.css";
 import App from "./App";
+import { I18nProvider } from "./i18n";
 
 createRoot(document.getElementById("root")!).render(
-  <BrowserRouter>
+  <I18nProvider>
     <App />
-  </BrowserRouter>,
+  </I18nProvider>,
 );