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]>

.env.example

@@ -117,20 +117,6 @@ LL2_API_TOKEN=
 # Optional: OpenSky credentials JSON file path (alternative to above)
 # OPENSKY_CREDENTIALS_FILE=/path/to/credentials.json
 
-# Optional: interface language pair (i18n). GEV_DEFAULT_LOCALE is the default
-# UI locale; GEV_SECONDARY_LOCALE is the second locale offered in the dock
-# language switch. English is ALWAYS shipped as the fallback catalog and is
-# resolvable no matter the pair. Only 'en' ships in this PR: both values are
-# validated against the shipped catalogs, so a secondary without a shipped
-# catalog degenerates the switch to a single English button (and an invalid,
-# unshipped, or degenerate pair falls back to that en-only shape with a
-# dev-server-only console.warn). Later locale PRs re-open the pair — see
-# docs/TRANSLATORS.md. CLIENT-EXPOSED (non-secret): both values reach the
-# browser bundle via import.meta.env defines — do not put anything sensitive
-# here.
-# GEV_DEFAULT_LOCALE=en
-# GEV_SECONDARY_LOCALE=es
-
 # Optional: dev server config
 PORT=4173
 # Network binding. Default is localhost — the server is reachable ONLY from