refactor(i18n): key messages by English text; add Russian

Replace the semantic keys with the English source text, so code reads as the
UI does and English needs no catalog: t('Save keys'), tn(count, one, other)
for plurals, msg() for text stored in data, and bare data-i18n* markup that
translates the element's own English. Other languages are one flat JSON each.

- Simplify src/i18n: drop namespaces, the configurable locale pair
  (GEV_DEFAULT_LOCALE / GEV_SECONDARY_LOCALE) and its build defines. Locale
  comes from ?lang=, the stored choice, then browser languages.
- Add scripts/i18n.mjs (npm run i18n): extracts every message from t/tn/msg
  and markup, and fails on missing, unused, placeholder-mismatched or
  wrongly-shaped plural entries. npm test runs it via catalog.test.mjs.
- Add Russian (src/i18n/locales/ru.json, 833 messages), started from the
  community catalog in #312 and reviewed against the current English.
- Fix data-i18n placed on parents that would wipe their children (cockpit
  LEVEL and local weather cells, first-run EXPLORE MANUALLY tile), and a
  doubled degree sign in the TRACK ACQUIRED signal.
- Rewrite docs/TRANSLATORS.md for the new workflow; drop sections the earlier
  merge resurrected in docs/CURRENT-STATE.md.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

build/vite.js

@@ -52,8 +52,6 @@ export function createBrowserViteConfig({
   publicDir,
   googleApiKey,
   cesiumToken,
-  defaultLocale = '',
-  secondaryLocale = '',
   mapillaryToken,
   host = 'localhost',
   port = 4173,
@@ -109,14 +107,6 @@ export function createBrowserViteConfig({
     define: {
       'import.meta.env.GOOGLE_MAPS_API_KEY': JSON.stringify(googleApiKey),
       'import.meta.env.CESIUM_ION_TOKEN': JSON.stringify(cesiumToken),
-      // Locale pair (i18n): the client's default + secondary UI locale. These
-      // are NOT secrets and are read by src/i18n/locale.js, which validates
-      // them against the shipped catalogs and falls back to the built-in
-      // en-only pair (while English is the only shipped catalog) for
-      // empty/invalid/unshipped values. Explicit define entries are required
-      // because Vite only auto-exposes VITE_-prefixed vars on import.meta.env.
-      'import.meta.env.GEV_DEFAULT_LOCALE': JSON.stringify(defaultLocale),
-      'import.meta.env.GEV_SECONDARY_LOCALE': JSON.stringify(secondaryLocale),
       // Mapillary client tokens are public by design (see SECURITY.md).
       'import.meta.env.MAPILLARY_CLIENT_TOKEN': JSON.stringify(
         mapillaryToken ?? '',