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/sceneControls.js

@@ -58,7 +58,7 @@ export class SceneControls {
       if (!this.destroyed) elements.file.value = '';
     });
     if (!subscribe) {
-      this.updateStatus(t('setup.scenes.statusReady'));
+      this.updateStatus(t('Ready'));
       this.setProgress(0);
       this.setButtons(false);
     }
@@ -124,7 +124,7 @@ export class SceneControls {
     const generation = ++this.actionGeneration;
     const failed = () => {
       if (!this.destroyed && generation === this.actionGeneration)
-        this.updateStatus(t('setup.scenes.status.actionFailed'));
+        this.updateStatus(t('Scene action failed'));
     };
     try {
       const result = this.actions[action](...args);
@@ -163,9 +163,7 @@ export class SceneControls {
     const shot = scene?.shots.find((item) => item.id === shotId);
     if (
       shot &&
-      window.confirm(
-        t('setup.scenes.status.deleteShotConfirm', { shot: shot.title }),
-      )
+      window.confirm(t('Delete shot "{shot}"?', { shot: shot.title }))
     )
       this.run('deleteShot', sceneId, shotId);
   }