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/engine/orchestrator.ts

@@ -64,6 +64,7 @@ export async function startSession(
     styleReferenceImage: req.styleReferenceImage?.trim() || undefined,
     orientation: coerceOrientation(req.orientation),
     playerName: req.playerName?.trim() || undefined,
+    language: req.language?.trim() || undefined,
   };
 
   // Stage 0 — Architect (+ optional auto style selection, in parallel).