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/components/FigmaBanner.tsx

@@ -70,7 +70,7 @@ function internalPath(linkUrl: string): string | null {
 /**
  * Banner link URLs are stored unprefixed (e.g. `/browse?post=123`). When the
  * viewer is on a non-English locale we must re-prefix them with the active
- * language path (`/malay/browse?post=123`) so navigation doesn't drop into
+ * language path (`/ms/browse?post=123`) so navigation doesn't drop into
  * the English version of the post.
  */
 function localizeLinkUrl(linkUrl: string, lang: Lang): string {