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/layers/firms/queries.js

@@ -60,24 +60,24 @@ export function createQueries({
     getStats() {
       const now = Date.now();
       const staleText = layerState._lastUpdate
-        ? t('layers.firms.staleCached', {
+        ? t('STALE · cached {age}', {
             age:
               components.model.formatAge(now - layerState._lastUpdate) || '<1h',
           })
-        : t('layers.status.stale');
+        : t('STALE');
       let loadingLabel = '';
       if (layerState._loading) {
         loadingLabel = layerState._fires.length
-          ? t('layers.meta.refreshing')
-          : t('layers.meta.loading');
+          ? t('refreshing...')
+          : t('loading...');
       } else if (layerState._keyRequired) {
-        loadingLabel = t('layers.firms.keyRequired');
+        loadingLabel = t('KEY REQUIRED');
       } else if (layerState._stale) {
         loadingLabel = staleText;
       } else if (layerState._error) {
         loadingLabel = layerState._error;
       } else if (layerState._lastUpdate) {
-        loadingLabel = t('layers.firms.liveUpdated', {
+        loadingLabel = t('LIVE · updated {age}', {
           age: components.model.formatAgoMinutes(now - layerState._lastUpdate),
         });
       }
@@ -93,7 +93,7 @@ export function createQueries({
         // the operator can take.
         keyRequired: layerState._keyRequired,
         error: layerState._keyRequired
-          ? t('layers.firms.keyRequired')
+          ? t('KEY REQUIRED')
           : layerState._stale
             ? staleText
             : layerState._error,