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/ui/locationControls.js

@@ -151,16 +151,12 @@ export class LocationControls {
     // localized static readout (resolved place names stay verbatim data).
     this.elements.statusCity.textContent =
       lines.city === LOCATION_MINI_EMPTY.city
-        ? t('cockpit.location.miniCityInitial')
+        ? t('📍 Location: --')
         : lines.city;
     if (lines.poi === LOCATION_MINI_EMPTY.poi) {
-      this.elements.statusPoi.textContent = t(
-        'cockpit.location.miniPoiInitial',
-      );
+      this.elements.statusPoi.textContent = t('Landmark: --');
     } else if (lines.poi === LOCATION_MINI_SEARCHED_PLACEHOLDER.poi) {
-      this.elements.statusPoi.textContent = t(
-        'cockpit.location.miniSearchedPlaceholder',
-      );
+      this.elements.statusPoi.textContent = t('Searched location');
     } else {
       this.elements.statusPoi.textContent = lines.poi;
     }
@@ -173,7 +169,7 @@ export class LocationControls {
       const icon = this.doc.createElement('span');
       icon.className = 'orbit-icon';
       icon.textContent = '↻';
-      this.orbitIndicator.append(icon, ` ${t('cockpit.location.orbitLabel')}`);
+      this.orbitIndicator.append(icon, ` ${t('ORBIT')}`);
       this.doc.body.appendChild(this.orbitIndicator);
     }
     return this.orbitIndicator;