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/contextLayerChanges.js

@@ -71,7 +71,7 @@ export function _handleContextLayerChange(change) {
             },
             {
               localizedMessage: t(
-                'cockpit.context.toastMissionsCancelRestoreFailed',
+                'Space Missions cancellation could not restore the previous layer state',
               ),
             },
           ),
@@ -94,18 +94,17 @@ export function _handleContextLayerChange(change) {
       !this._userFacingContextNotificationTokens.has(change.notificationToken)
     ) {
       this.showToast(
-        change.reason || t('cockpit.context.toastLayerUnavailable'),
+        change.reason ||
+          t('That layer is unavailable in the current Context mode'),
       );
     }
     this._syncContextModeButtons();
     return;
   }
   if (change?.type === 'visibility-failed') {
-    const failureMessage = t('cockpit.context.toastLayerLifecycleFailed', {
+    const failureMessage = t('{layerId} could not {action} cleanly', {
       layerId: change.layerId,
-      action: change.enabled
-        ? t('cockpit.context.actionStart')
-        : t('cockpit.context.actionStop'),
+      action: change.enabled ? t('start') : t('stop'),
     });
     // A failed direct Context-shell START has already had its siblings
     // cleared by the visibility guard. Wait outside the synchronous manager