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/data/satelliteClass.js

@@ -17,7 +17,7 @@
  * for), then the constellations users can actually name, then the grab-bag.
  */
 
-import { t } from '../i18n/index.js';
+import { t, msg } from '../i18n/index.js';
 
 /**
  * Class registry. `label` is the card/legend token, `color` the point color,
@@ -84,24 +84,26 @@ export const SATELLITE_CLASS_ORDER = Object.freeze([
 // translate through these keys at the presentation boundary.
 const CLASS_DISPLAY_KEYS = Object.freeze({
   station: Object.freeze({
-    label: 'layers.satellites.class.station',
-    blurb: 'layers.satellites.class.stationBlurb',
+    label: msg('STATION'),
+    blurb: msg('Crewed stations and their visiting vehicles'),
   }),
   nav: Object.freeze({
-    label: 'layers.satellites.class.nav',
-    blurb: 'layers.satellites.class.navBlurb',
+    label: msg('NAV'),
+    blurb: msg('GNSS navigation — GPS, GLONASS, Galileo'),
   }),
   geo: Object.freeze({
-    label: 'layers.satellites.class.geo',
-    blurb: 'layers.satellites.class.geoBlurb',
+    label: msg('GEO'),
+    blurb: msg(
+      'Geostationary belt — comms and weather, fixed over the equator',
+    ),
   }),
   visual: Object.freeze({
-    label: 'layers.satellites.class.visual',
-    blurb: 'layers.satellites.class.visualBlurb',
+    label: msg('VISUAL'),
+    blurb: msg('Brightest naked-eye objects — CelesTrak visual group'),
   }),
   comms: Object.freeze({
-    label: 'layers.satellites.class.comms',
-    blurb: 'layers.satellites.class.commsBlurb',
+    label: msg('COMMS'),
+    blurb: msg('Broadband constellation shell — shown only in DENSE mode'),
   }),
 });