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/frontend/src/lib/i18n/locales/en.ts

@@ -617,6 +617,7 @@ const en = {
 	"game.battle.not_found": "battle not found",
 	"game.battle.back_to_report": "back to report",
 	"game.battle.back_to_map": "back to map",
+	"game.battle.scene_label": "battle scene",
 	"game.battle.controls.play": "play",
 	"game.battle.controls.pause": "pause",
 	"game.battle.controls.step_forward": "step forward",

ui/frontend/src/lib/i18n/locales/ru.ts

@@ -621,6 +621,7 @@ const ru: Record<keyof typeof en, string> = {
 	"game.battle.not_found": "сражение не найдено",
 	"game.battle.back_to_report": "к отчёту",
 	"game.battle.back_to_map": "к карте",
+	"game.battle.scene_label": "сцена боя",
 	"game.battle.controls.play": "запустить",
 	"game.battle.controls.pause": "пауза",
 	"game.battle.controls.step_forward": "шаг вперёд",