feat(i18n): add language switcher with en/ja translations

- New client-side i18n via React Context (useI18n, tArray, I18nProvider)
- Catalog ships 21 locale stubs; only zh-CN/en/ja have reviewed translations
- Header language switcher (globe icon + short label) before settings gear
- All hardcoded Chinese UI text migrated to keys: typewriter, options,
  hints (with embedded gear icon via dangerouslySetInnerHTML), settings
  panel, footer/about, play page hints
- AI output language follows user-selected locale via trailing one-liner
  directive appended to Architect/Writer/CharacterDesigner/InsertBeat
  user messages (preserves system-prompt cacheability)
- Per-locale separator rule: zh uses middot between every glyph; en/ja
  use plain spaces
- Option value → i18n key suffix maps preserve Chinese as the underlying
  identifier so analytics unions and STYLE_MAP keys stay byte-stable

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

lib/types/index.ts

@@ -309,6 +309,14 @@ export type Session = {
    * only (localStorage); never persisted server-side.
    */
   playerName?: string;
+  /**
+   * Active UI locale when the session was started, in BCP-47 form (e.g.
+   * "zh-CN", "en", "ja"). The engine appends a single-line language directive
+   * to the Architect / Writer user messages so AI-generated dialogue, beats,
+   * and narration are produced in this language. Absent → "zh-CN" for
+   * back-compat with sessions created before this field existed.
+   */
+  language?: string;
 };
 
 // ──────────────────────────────────────────────────────────────────────
@@ -428,6 +436,9 @@ export type StartRequest = {
   orientation?: Orientation;
   /** Optional player display name — see Session.playerName. */
   playerName?: string;
+  /** Active UI locale — see Session.language. Drives the engine's language
+   *  directive so AI output is generated in the player's chosen language. */
+  language?: string;
 };
 
 // /api/parse-style-image — vision LLM extracts a textual painting-style