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/layers/launches/policyHelpers.js

@@ -77,14 +77,14 @@ export function createPolicyHelpers({
       : 0;
     return {
       orbit: orbitName
-        ? `${orbitAllowed ? '' : t('layers.missions.rows.plannedPrefix')}${orbitName}`
+        ? `${orbitAllowed ? '' : t('PLANNED · ')}${orbitName}`
         : null,
       ascent:
         suppliedTrajectoryPoints > 1
-          ? t('layers.missions.rows.suppliedTrajectory')
+          ? t('SUPPLIED TRAJECTORY POINTS')
           : replayAvailable
-            ? t('layers.missions.rows.reconstructedEstimate')
-            : t('layers.missions.rows.unavailable'),
+            ? t('RECONSTRUCTED ESTIMATE')
+            : t('UNAVAILABLE'),
       replayAvailable: Boolean(replayAvailable),
     };
   }
@@ -198,7 +198,7 @@ export function createPolicyHelpers({
   function formatMissionEventTime(launchTime) {
     const date = new Date(launchTime);
     if (!launchTime || !Number.isFinite(date.getTime()))
-      return t('layers.missions.rows.unavailable');
+      return t('UNAVAILABLE');
     return `${date.toISOString().slice(0, 10)}\n${date.toISOString().slice(11, 19)} UTC`;
   }