feat(ui): locale persistence + i18n completeness guards (F3)

An audit found the client already i18n-first: one hard-coded UI string
(the battle-scene aria-label, now keyed) and en/ru already share an
identical 692-key set.

- Persist the locale: i18n.setLocale writes localStorage (galaxy-locale)
  and the store boots from stored > browser detection > default, so a
  language switch survives reloads.
- tests/i18n-completeness.test.ts: en/ru key-set parity, non-empty
  values, and locale persistence.
- Docs: ui/docs/i18n.md; mark F3 done in ui/PLAN-finalize.md.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

ui/PLAN-finalize.md

@@ -69,7 +69,17 @@ axe-core scan; full keyboard reachability with visible focus.
 Tests: axe-core integration tests on every top-level view; Playwright
 keyboard-only navigation.
 
-## F3 — Localisation completeness
+## F3 — Localisation completeness — done
+
+An audit found the client already i18n-first (the single hard-coded UI
+string, the battle-scene `aria-label`, is now keyed); en/ru already have
+identical key sets (692 keys). Added locale **persistence** to the i18n
+store (`localStorage` `galaxy-locale`: stored choice > browser detection
+> default) so a switch survives reloads, and
+`tests/i18n-completeness.test.ts` enforcing en/ru key-set parity,
+non-empty values, and persistence. A noisy literal-text lint was
+deliberately skipped — the structural parity test plus the i18n-first
+convention cover drift. Docs: `ui/docs/i18n.md`.
 
 (From Phase 35.) Goal: every visible string is translated (en + ru).