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/shareRestoration.js

@@ -189,12 +189,10 @@ export class ShareRestoration {
     if (result.classification === 'pending') {
       this._shareTrackingNoticeGeneration += 1;
       this._shareTrackingAcquiringKey = trackingKey;
-      this.showStatus(t('cockpit.status.acquiring'), {
+      this.showStatus(t('ACQUIRING'), {
         state: 'acquiring',
-        detail: t('cockpit.status.sharedSubjectDetail', {
-          subject: String(
-            result.label || t('cockpit.status.subjectFallback'),
-          ).toUpperCase(),
+        detail: t('SHARED {subject}', {
+          subject: String(result.label || t('entity')).toUpperCase(),
         }),
         persistent: true,
       });
@@ -219,13 +217,15 @@ export class ShareRestoration {
     const noticeGeneration = ownsAcquiringNotice
       ? this._shareTrackingNoticeGeneration
       : ++this._shareTrackingNoticeGeneration;
-    const subject = result.label || t('cockpit.status.subjectFallback');
+    const subject = result.label || t('entity');
     const message =
       result.classification === 'expired'
-        ? t('cockpit.status.sharedFollowExpired', { subject })
+        ? t('Shared {subject} follow expired', { subject })
         : result.classification === 'source-unavailable'
-          ? t('cockpit.status.sharedRestoreFailed', { subject })
-          : t('cockpit.status.sharedUnavailable', { subject });
+          ? t('Shared {subject} could not be restored — feed unavailable', {
+              subject,
+            })
+          : t('Shared {subject} is unavailable', { subject });
     const showAfterStartupCover = () => {
       this._lifetime.frame(() => {
         if (