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.
