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/cockpitTrackingController.js

@@ -59,8 +59,8 @@ export function syncTr3bToggle(info) {
   this.tr3bToggle.hidden = !icao24;
   this.tr3bToggle.setAttribute('aria-pressed', converted ? 'true' : 'false');
   this.tr3bToggle.title = converted
-    ? t('cockpit.context.tr3bRestoreTitle')
-    : t('cockpit.context.tr3bTitle');
+    ? t('Restore real aircraft')
+    : t('Reclassify as TR-3B');
 }
 
 export function syncEntry() {
@@ -184,20 +184,17 @@ export function enter() {
   this.showBriefPage(0);
   this.startBriefRotation();
   const trackLabel =
-    info.callsign ||
-    info.registration ||
-    info.icao24 ||
-    t('cockpit.hud.fallbackCallsign');
+    info.callsign || info.registration || info.icao24 || t('AIRCRAFT');
   const trackHeading = String(
     Math.round(normalizeHeading(info.track ?? 0)),
   ).padStart(3, '0');
   this.pushCockpitSignal(
     'track',
     'track',
-    t('cockpit.signal.trackAcquired'),
-    t('cockpit.signal.trackDetail', {
+    t('TRACK ACQUIRED'),
+    t('{label} · COURSE {heading}°', {
       label: trackLabel,
-      heading: `${trackHeading}°`,
+      heading: trackHeading,
     }),
   );
   this.updateHud(info, performance.now(), true);