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/App.tsx

@@ -1,4 +1,11 @@
-import { BrowserRouter, Navigate, Route, Routes } from "react-router-dom";
+import {
+  BrowserRouter,
+  Navigate,
+  Route,
+  Routes,
+  useLocation,
+  useParams,
+} from "react-router-dom";
 import { I18nProvider } from "./i18n";
 import { MotionProvider } from "./motion";
 import { ToastProvider } from "./components/Toast";
@@ -19,7 +26,20 @@ import { AdminRouteTree } from "./adminRouteTree";
 import { AdminRouterModeProvider } from "./adminRouterMode";
 import { ImageLightboxProvider } from "./components/messageStream/overlays/ImageLightbox";
 import { VideoPlayerProvider } from "./components/messageStream/overlays/VideoPlayer";
-import { localizedHomeRoutes } from "./languageRoutes";
+import { legacyLanguageRedirects, localizedHomeRoutes } from "./languageRoutes";
+
+/**
+ * Redirects shared links that still use the old long-form language prefix
+ * (e.g. /chinese, /malay/browse) to the new short codes (/cn, /ms/browse).
+ * Preserves the sub-path, query string, and hash.
+ */
+function LegacyLangRedirect({ to }: { to: string }) {
+  const params = useParams();
+  const { search, hash } = useLocation();
+  const splat = params["*"];
+  const sub = splat ? `/${splat}` : "";
+  return <Navigate to={`${to}${sub}${search}${hash}`} replace />;
+}
 
 const adminEnabled = import.meta.env.VITE_DISABLE_ADMIN !== "true";
 
@@ -94,6 +114,21 @@ export default function App() {
                           ))}
                         </Route>
 
+                        {/* Legacy long-form language URLs → short-code
+                            redirects. Shared links (e.g. WeChat) keep working. */}
+                        {legacyLanguageRedirects.map((redirect) => (
+                          <Route key={redirect.from}>
+                            <Route
+                              path={redirect.from}
+                              element={<LegacyLangRedirect to={redirect.to} />}
+                            />
+                            <Route
+                              path={`${redirect.from}/*`}
+                              element={<LegacyLangRedirect to={redirect.to} />}
+                            />
+                          </Route>
+                        ))}
+
                         {adminEnabled ? (
                           AdminRouteTree()
                         ) : (