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/satellites/controls.js

@@ -494,22 +494,27 @@ export function createControls({ state: layerState, services, parts, source }) {
       const active =
         layerState._params.catalog === 'dense' &&
         layerState._denseStatus === 'ready';
-      let title = t('layers.satellites.denseTitleAdd');
-      if (loading) title = t('layers.satellites.denseTitleLoading');
+      let title = t(
+        'Add the full Starlink broadband shell (thousands of extra points)',
+      );
+      if (loading) title = t('Loading the Starlink shell…');
       else if (failed)
-        title = t('layers.satellites.denseTitleFailed', {
-          detail: layerState._denseError || t('layers.satellites.loadFailed'),
+        title = t('Starlink {detail} — click to retry', {
+          detail: layerState._denseError || t('load failed'),
         });
-      else if (active) title = t('layers.satellites.denseTitleActive');
+      else if (active)
+        title = t(
+          'Showing the full Starlink shell — click for the core catalog only',
+        );
       return {
         chips: [
           {
             id: 'catalog',
             label: loading
-              ? t('layers.satellites.denseChipLoading')
+              ? t('DENSE ···')
               : failed
-                ? t('layers.satellites.denseChipFailed')
-                : t('layers.satellites.denseChip'),
+                ? t('DENSE ✕')
+                : t('DENSE'),
             active,
             busy: loading,
             disabled: loading,