Switch language in place, without a reload

Choosing a language in the picker now applies it at once. The choice is
stored and ?lang is kept in step through replaceState, and the picker keeps
focus.

Code that writes translated text itself re-presents it on a switch:
onLocaleChange() lets an owner listen, removed at teardown, and status
writers hand over a function of the locale (voice cards, scene director,
SDR, draw hints, imagery split, map sources, Bhote Koshi media) so the
words are rebuilt rather than re-sent. The HUD, cockpit, radio, CCTV,
launch, awareness and credit texts follow the same hook, and
applyDocumentTranslations can be applied again without undoing text code
wrote since.

The unsaved-work registry and persistLocaleAndReload existed only to guard
the reload, so they go, with their catalog entries.

docs/TRANSLATORS.md

@@ -22,7 +22,7 @@ Every other language is one flat JSON file, `src/i18n/locales/<code>.json`, keye
 
 A `tn()` entry is keyed by the English plural form and holds one string per `Intl.PluralRules` category of that language. A missing entry falls back to English.
 
-The page picks its language from `?lang=`, then the choice stored by the language picker (the globe button in the bottom-right corner), then the browser languages, then English. `?lang=` is not stored, and copied share links leave it out. Switching reloads the page.
+The page picks its language from `?lang=`, then the choice stored by the language picker (the globe button in the bottom-right corner), then the browser languages, then English. `?lang=` is not stored, and copied share links leave it out. Switching applies at once, without a reload.
 
 ## The check