Locales now load only when needed

The admin app used to ship every locale's translation JSON in the entry bundle that loads on first paint. That meant the sign-in page was paying for both English and Spanish translations — about 70KB of strings for content the user would never see — before anything rendered.

Locales are now loaded lazily, each in its own async chunk that's fetched only when needed. The active locale lands first, others wait until the user actually switches.

As we add more languages, this approach scales — every new locale becomes a separate async chunk instead of inflating the entry bundle for everyone.