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/contextActions.js

@@ -1,4 +1,4 @@
-import { t } from '../i18n/index.js';
+import { t, tn } from '../i18n/index.js';
 import {
   settleUserFacingContextAction,
   contextModeWord,
@@ -6,7 +6,7 @@ import {
 
 export async function _runUserFacingContextAction(
   operation,
-  message = t('cockpit.context.toastRestoreFailed'),
+  message = t('Context could not restore every layer; try again'),
   { falseIsFailure = true, localizedMessage = null } = {},
 ) {
   if (this.destroyed) return false;
@@ -223,25 +223,29 @@ export function clearSelectedLayers() {
   const operation = managerOperation
     .then((result) => {
       if (result.targetIds.length === 0) {
-        this.showToast(t('layers.clear.toast.noneSelected'));
+        this.showToast(t('No selected data layers'));
       } else if (result.notClearedIds.length > 0) {
         this.showToast(
-          t('layers.clear.toast.notCleared', {
-            count: result.notClearedIds.length,
-          }),
+          tn(
+            result.notClearedIds.length,
+            '{count} data layer could not be cleared',
+            '{count} data layers could not be cleared',
+          ),
         );
       } else {
         this.showToast(
-          t('layers.clear.toast.cleared', {
-            count: result.clearedIds.length,
-          }),
+          tn(
+            result.clearedIds.length,
+            'Cleared {count} data layer',
+            'Cleared {count} data layers',
+          ),
         );
       }
       return result;
     })
     .catch((error) => {
       console.warn('[Data] clear selected layers failed', error);
-      this.showToast(t('cockpit.actions.clearLayersFailedToast'));
+      this.showToast(t('Selected data layers could not be cleared'));
       return {
         targetIds: [],
         items: [],