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/index.svelte.ts

@@ -106,15 +106,36 @@ function isLocale(value: string): value is Locale {
 	return SUPPORTED_LOCALES.some((entry) => entry.code === value);
 }
 
+/** `localStorage` key holding the user's explicit locale choice. */
+export const LOCALE_STORAGE_KEY = "galaxy-locale";
+
+function readStoredLocale(): Locale | null {
+	if (typeof localStorage === "undefined") return null;
+	const value = localStorage.getItem(LOCALE_STORAGE_KEY);
+	return value !== null && isLocale(value) ? value : null;
+}
+
+/**
+ * initialLocale resolves the boot locale: an explicit stored choice
+ * wins, otherwise the browser/system preference, otherwise the default.
+ */
+function initialLocale(): Locale {
+	return readStoredLocale() ?? detectInitialLocale();
+}
+
 class I18nStore {
-	locale: Locale = $state(detectInitialLocale());
+	locale: Locale = $state(initialLocale());
 
 	/**
-	 * setLocale changes the active locale. Components reading
+	 * setLocale changes the active locale and persists the choice to
+	 * `localStorage`, so it survives a reload. Components reading
 	 * `i18n.t(...)` re-render automatically through the rune.
 	 */
 	setLocale(next: Locale): void {
 		this.locale = next;
+		if (typeof localStorage !== "undefined") {
+			localStorage.setItem(LOCALE_STORAGE_KEY, next);
+		}
 	}
 
 	/**