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/templates/welcome.html

@@ -5,9 +5,9 @@
   <aside id="first-run-launcher" role="dialog" aria-labelledby="first-run-title" aria-describedby="first-run-description" hidden>
     <div class="first-run-scanline" aria-hidden="true"></div>
     <header class="first-run-header">
-      <span class="first-run-kicker"  data-i18n="setup.firstRun.kicker">MISSION CONTROL · FIRST LAUNCH</span>
+      <span class="first-run-kicker" data-i18n>MISSION CONTROL · FIRST LAUNCH</span>
     </header>
-    <h2 id="first-run-title"  data-i18n="setup.firstRun.title">Choose your first view</h2>
+    <h2 id="first-run-title" data-i18n>Choose your first view</h2>
     <!-- The card's one persuasive line, owner-authored and VERBATIM (2026-08-23)
          — including the unspaced em dash. The arc it has to carry: this looks
          like something you should not have access to → the sources are all
@@ -16,12 +16,12 @@
     <div class="first-run-choices">
       <button type="button" data-first-run-choice="contacts">
         <span class="material-symbols-outlined" aria-hidden="true">radar</span>
-        <span><strong data-i18n="setup.firstRun.choice.contacts">LIVE CONTACTS</strong><small data-i18n="setup.firstRun.choice.contactsSub">Aircraft, vessels and nearby intelligence</small></span>
+        <span><strong data-i18n>LIVE CONTACTS</strong><small data-i18n>Aircraft, vessels and nearby intelligence</small></span>
         <span class="material-symbols-outlined first-run-arrow" aria-hidden="true">arrow_forward</span>
       </button>
       <button type="button" data-first-run-choice="space-missions">
         <span class="material-symbols-outlined" aria-hidden="true">rocket_launch</span>
-        <span><strong data-i18n="setup.firstRun.choice.spaceMissions">SPACE MISSIONS</strong><small data-i18n="setup.firstRun.choice.spaceMissionsSub">Launches, spacecraft and orbital context</small></span>
+        <span><strong data-i18n>SPACE MISSIONS</strong><small data-i18n>Launches, spacecraft and orbital context</small></span>
         <span class="material-symbols-outlined first-run-arrow" aria-hidden="true">arrow_forward</span>
       </button>
       <button type="button" data-first-run-choice="environmental">
@@ -35,18 +35,18 @@
       </button>
       <button type="button" class="first-run-explore" data-first-run-choice="explore">
         <span class="material-symbols-outlined" aria-hidden="true">public</span>
-        <span  data-i18n="setup.firstRun.choice.explore"  data-i18n="setup.firstRun.choice.exploreSub"><strong>EXPLORE MANUALLY</strong><small>Begin with a clean globe</small></span>
+        <span><strong data-i18n>EXPLORE MANUALLY</strong><small data-i18n>Begin with a clean globe</small></span>
         <span class="material-symbols-outlined first-run-arrow" aria-hidden="true">arrow_forward</span>
       </button>
     </div>
     <div class="first-run-footer">
       <label class="first-run-suppress">
         <input type="checkbox" data-first-run-suppress />
-        <span  data-i18n="setup.firstRun.suppress">Don't show this again</span>
+        <span data-i18n>Don't show this again</span>
       </label>
-      <span  data-i18n="setup.firstRun.dismissHint">ESC to dismiss</span>
+      <span data-i18n>ESC to dismiss</span>
     </div>
     <!-- Kept out of the footer row: it swaps to mission progress and errors, and
          a live region that also holds layout makes the row jump as it speaks. -->
-    <p class="first-run-note" data-first-run-status role="status" aria-live="polite"  data-i18n="setup.firstRun.note">Tip: the GEV MIC button in the dock lets you talk to the map.</p>
+    <p class="first-run-note" data-first-run-status role="status" aria-live="polite" data-i18n>Tip: the GEV MIC button in the dock lets you talk to the map.</p>
   </aside>