fix(i18n): overhaul i18n with [locale] routing, SSR translations, and hreflang SEO

Rewrites the i18n system introduced in PR #94 to use Next.js App Router
[locale] dynamic segments with SSR-rendered translations and proper
middleware locale routing.

- Add middleware locale detection: / rewrites to /zh-CN/ internally,
  /en and /ja pass through, /zh-CN/... redirects to bare path
- Move all 7 pages under app/[locale]/ with SSR translation injection
- Fix server→client serialization: pre-evaluate function-valued
  translations (makeSerializable) to eliminate hydration flash
- Fix language switch key flash: use hard navigation with localStorage-
  only persistence, avoiding React state update before page reload
- Add <link rel="alternate" hreflang> tags for multilingual SEO
- Fix Supabase setAll overwriting locale rewrite response
- Trim locales from 22 to 3 (zh-CN/en/ja), delete 19 incomplete files
- LLM-translate 240 firstact game preset JSONs (en + ja, landscape +
  portrait) and story titles via gemini-3.5-flash
- Delete 11 one-off migration scripts and outdated i18n docs
- Add useLocalePath hook and navigation utilities

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

lib/engine/orchestrator.ts

@@ -24,6 +24,7 @@ import { selectStyle } from "./agents/styleSelector";
 import { directInsertBeat, directScene } from "./director";
 import { STYLE_MAP } from "@/lib/options";
 import { parseJsonLoose } from "./jsonParser";
+import { isValidLocale } from "@/lib/i18n/utils";
 import {
   FREEFORM_CLASSIFY_SYSTEM,
   buildFreeformClassifyUserMessage,
@@ -65,7 +66,7 @@ export async function startSession(
     styleReferenceImage: req.styleReferenceImage?.trim() || undefined,
     orientation: coerceOrientation(req.orientation),
     playerName: req.playerName?.trim() || undefined,
-    language: req.language?.trim() || undefined,
+    language: (req.language?.trim() && isValidLocale(req.language.trim())) ? req.language.trim() : undefined,
   };
 
   // Stage 0 — optional auto style selection. The story bible is no longer