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]>

src/tooling/viteBuild.test.mjs

@@ -11,8 +11,6 @@ test('explicit build inputs preserve browser-only defines, plugin order and loop
     plugins: [plugin],
     googleApiKey: 'browser-fixture',
     cesiumToken: 'ion-fixture',
-    defaultLocale: 'fr',
-    secondaryLocale: 'en',
   });
   // The Host check runs before every other plugin's middleware.
   assert.equal(config.plugins[0].name, 'host-check');
@@ -40,9 +38,6 @@ test('explicit build inputs preserve browser-only defines, plugin order and loop
   assert.deepEqual(config.define, {
     'import.meta.env.GOOGLE_MAPS_API_KEY': '"browser-fixture"',
     'import.meta.env.CESIUM_ION_TOKEN': '"ion-fixture"',
-    // Locale pair (i18n): threaded like the other explicit browser inputs.
-    'import.meta.env.GEV_DEFAULT_LOCALE': '"fr"',
-    'import.meta.env.GEV_SECONDARY_LOCALE': '"en"',
     'import.meta.env.MAPILLARY_CLIENT_TOKEN': '""',
   });
   assert.equal(