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/contextBindings.js

@@ -33,7 +33,7 @@ export function _initGlobalContextPanel() {
     void this._runUserFacingContextAction(
       (notificationToken) =>
         this._selectContextMode(nextMode, { notificationToken }),
-      t('cockpit.context.toastTransitionFailedContacts'),
+      t('Contacts could not complete the requested transition; try again'),
     ).then((succeeded) => {
       if (
         !this.destroyed &&
@@ -62,7 +62,9 @@ export function _initGlobalContextPanel() {
     void this._runUserFacingContextAction(
       (notificationToken) =>
         this._selectContextMode(nextMode, { notificationToken }),
-      t('cockpit.context.toastTransitionFailedMissions'),
+      t(
+        'Space Missions could not complete the requested transition; try again',
+      ),
     ).then((succeeded) => {
       if (
         !this.destroyed &&
@@ -109,14 +111,16 @@ export function _initGlobalContextPanel() {
       this.showToast(
         stats?.statusMessage ||
           (stats?.status === 'zoom-in'
-            ? t('cockpit.context.toastZoomToSearch')
-            : t('cockpit.context.toastInstallationsRefreshed')),
+            ? t('Zoom in to search mapped installations')
+            : t('Nearby installations refreshed')),
       );
       return true;
-    }, t('cockpit.context.toastInstallationsRefreshFailed')).finally(() => {
-      if (this.destroyed) return;
-      button.setAttribute('aria-disabled', 'false');
-      button.setAttribute('aria-busy', 'false');
-    });
+    }, t('Nearby installations could not be refreshed; try again')).finally(
+      () => {
+        if (this.destroyed) return;
+        button.setAttribute('aria-disabled', 'false');
+        button.setAttribute('aria-busy', 'false');
+      },
+    );
   });
 }