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/components/LanguageSwitcher.tsx

@@ -0,0 +1,27 @@
+import { useI18n } from "@/i18n/context";
+
+/**
+ * Compact language toggle — shows a clickable flag that switches between
+ * English and Chinese.  Persists choice to localStorage.
+ */
+export function LanguageSwitcher() {
+  const { locale, setLocale, t } = useI18n();
+
+  const toggle = () => setLocale(locale === "en" ? "zh" : "en");
+
+  return (
+    <button
+      type="button"
+      onClick={toggle}
+      className="group relative inline-flex items-center gap-1.5 px-2 py-1 text-xs text-muted-foreground hover:text-foreground transition-colors cursor-pointer focus-visible:outline-none focus-visible:ring-1 focus-visible:ring-ring rounded"
+      title={t.language.switchTo}
+      aria-label={t.language.switchTo}
+    >
+      {/* Show the *other* language's flag as the clickable target */}
+      <span className="text-base leading-none">{locale === "en" ? "🇨🇳" : "🇬🇧"}</span>
+      <span className="hidden sm:inline font-display tracking-wide uppercase text-[0.65rem]">
+        {locale === "en" ? "中文" : "EN"}
+      </span>
+    </button>
+  );
+}