feat(routing): shorten language URL prefixes to ISO codes with legacy redirects

Rename the localized URL prefixes from full English names to short
ISO-style codes:
  /chinese    -> /cn
  /japanese   -> /ja
  /korean     -> /ko
  /vietnamese -> /vi
  /indonesian -> /id
  /malay      -> /ms

Add legacyLanguageRedirects mapping and a LegacyLangRedirect component
in App.tsx so links shared on WeChat (and elsewhere) that still use the
long-form paths keep landing on the right page. The redirect preserves
the sub-path, query string, and hash, e.g.
  /malay/browse?post=42#x -> /ms/browse?post=42#x

Also refresh doc-comment examples in i18n.tsx, FigmaBanner.tsx,
PublicLayout.tsx, and useLocalizedPath.ts so future readers see the new
prefixes.

src/layouts/PublicLayout.tsx

@@ -305,7 +305,7 @@ export function PublicLayout() {
   const nav = useNavigate();
   const lp = useLocalizedPath();
 
-  // Keep i18n state in sync with URL so deep links (`/malay/browse`) flip the
+  // Keep i18n state in sync with URL so deep links (`/ms/browse`) flip the
   // UI language even if the user navigated via address bar or shared link.
   useEffect(() => {
     const urlLang = languageFromPathname(pathname);