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/i18n/context.tsx

@@ -0,0 +1,58 @@
+import { createContext, useContext, useState, useCallback, type ReactNode } from "react";
+import type { Locale, Translations } from "./types";
+import { en } from "./en";
+import { zh } from "./zh";
+
+const TRANSLATIONS: Record<Locale, Translations> = { en, zh };
+const STORAGE_KEY = "hermes-locale";
+
+function getInitialLocale(): Locale {
+  try {
+    const stored = localStorage.getItem(STORAGE_KEY);
+    if (stored === "en" || stored === "zh") return stored;
+  } catch {
+    // SSR or privacy mode
+  }
+  return "en";
+}
+
+interface I18nContextValue {
+  locale: Locale;
+  setLocale: (l: Locale) => void;
+  t: Translations;
+}
+
+const I18nContext = createContext<I18nContextValue>({
+  locale: "en",
+  setLocale: () => {},
+  t: en,
+});
+
+export function I18nProvider({ children }: { children: ReactNode }) {
+  const [locale, setLocaleState] = useState<Locale>(getInitialLocale);
+
+  const setLocale = useCallback((l: Locale) => {
+    setLocaleState(l);
+    try {
+      localStorage.setItem(STORAGE_KEY, l);
+    } catch {
+      // ignore
+    }
+  }, []);
+
+  const value: I18nContextValue = {
+    locale,
+    setLocale,
+    t: TRANSLATIONS[locale],
+  };
+
+  return (
+    <I18nContext.Provider value={value}>
+      {children}
+    </I18nContext.Provider>
+  );
+}
+
+export function useI18n() {
+  return useContext(I18nContext);
+}