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/mapStackChips.js

@@ -41,11 +41,11 @@ export function mapStackChipModel(stack, activeId) {
   // Stack labels are provider/dataset names (keep-English); the availability
   // sentence around them is copy and localizes here at the model boundary.
   const fallbackReason = requiresIon
-    ? t('setup.keySetup.requirement', {
+    ? t('Needs {envVars} — add it in Provider Settings', {
         envVars: keySetupRequirementEnvVars('cesium-ion'),
       })
-    : t('setup.mapStack.unavailableReason', {
-        label: label || t('setup.mapStack.fallbackName'),
+    : t('{label} is unavailable', {
+        label: label || t('This map stack'),
       });
   const unavailableHint = available
     ? ''
@@ -126,7 +126,7 @@ export function renderMapStackChips(
     if (!model.available) {
       chip.setAttribute(
         'aria-label',
-        t('setup.mapStack.unavailableAriaLabel', {
+        t('{label} unavailable: {hint}', {
           label: model.label,
           hint: model.unavailableHint,
         }),