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/voice/realtimeInput.js

@@ -119,10 +119,7 @@ export class RealtimeInput {
           this.ui.root.dataset.pushToTalk = 'held';
           this.setMicrophoneEnabled(true);
           if (this.status === 'listening')
-            this.setStatus(
-              'listening',
-              t('setup.voice.detail.releaseSpaceSend'),
-            );
+            this.setStatus('listening', t('Release Space to send'));
         } else {
           this.start({ pushToTalk: true });
           // start() performs a controlled stop() before connecting. Restore the
@@ -198,7 +195,7 @@ export class RealtimeInput {
     if (!this.pushToTalkMode) return;
     this.setMicrophoneEnabled(false);
     if (this.status === 'listening')
-      this.setStatus('listening', t('setup.voice.detail.holdSpaceTalk'));
+      this.setStatus('listening', t('Hold Space to talk'));
     else this.updateVoiceButtonLabel();
   }