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/contextSubscriptions.js

@@ -125,12 +125,15 @@ export function connectContextManager(manager) {
                 restoreError,
               );
             }
-            return t('cockpit.context.toastStartBlocked', {
-              mode:
-                entryMode === 'space-missions'
-                  ? t('cockpit.context.modeSpaceMissions')
-                  : t('cockpit.context.modeContext'),
-            });
+            return t(
+              '{mode} could not start because another layer did not stop cleanly',
+              {
+                mode:
+                  entryMode === 'space-missions'
+                    ? t('Space Missions')
+                    : t('Context'),
+              },
+            );
           } finally {
             if (ownsNotificationToken) {
               this._userFacingContextNotificationTokens.delete(