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/visualSettings.js

@@ -1191,8 +1191,8 @@ export class VisualSettings {
       this._celestialBtn.disabled = !styleSupported;
       this._celestialBtn.setAttribute('aria-disabled', String(!styleSupported));
       this._celestialBtn.title = styleSupported
-        ? t('cockpit.display.celestialToggleTitle')
-        : t('cockpit.display.celestialUnavailableTitle');
+        ? t('Celestial ring — reveal the full globe')
+        : t('Celestial ring — available in Normal style');
     }
     let cameraFocused = false;
     if (nextEnabled && focus) {
@@ -1649,31 +1649,23 @@ export class VisualSettings {
     btn.setAttribute(
       'aria-label',
       enabled
-        ? t('cockpit.display.detectionAriaTemplate', {
+        ? t('Detection overlay: {mode}', {
             mode: String(modeLabel).toLowerCase(),
           })
-        : t('cockpit.display.detectionAriaOff'),
+        : t('Detection overlay: off'),
     );
     btn.classList.remove('active', 'god', 'panoptic');
     if (modeLabel === 'SPARSE') {
-      btn.querySelector('.pp-label').textContent = t(
-        'cockpit.display.detectionLabelSparse',
-      );
+      btn.querySelector('.pp-label').textContent = t('SPARSE');
       btn.classList.add('active');
     } else if (modeLabel === 'BALANCED') {
-      btn.querySelector('.pp-label').textContent = t(
-        'cockpit.display.detectionLabelBalanced',
-      );
+      btn.querySelector('.pp-label').textContent = t('BALANCED');
       btn.classList.add('active');
     } else if (modeLabel === 'DENSE') {
-      btn.querySelector('.pp-label').textContent = t(
-        'cockpit.display.detectionLabelDense',
-      );
+      btn.querySelector('.pp-label').textContent = t('DENSE');
       btn.classList.add('active', 'panoptic');
     } else {
-      btn.querySelector('.pp-label').textContent = t(
-        'cockpit.display.detectionLabel',
-      );
+      btn.querySelector('.pp-label').textContent = t('DETECT');
     }
 
     if (this._detectionSliderRow) {