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- Delete 11 one-off migration scripts and outdated i18n docs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>

components/CustomForm.tsx

@@ -4,10 +4,12 @@ import { useRouter } from "next/navigation";
 import { useState } from "react";
 import { track } from "@/lib/analytics";
 import { useI18n } from "@/lib/i18n/client";
+import { useLocalePath } from "@/lib/i18n/hooks";
 
 export function CustomForm() {
   const router = useRouter();
   const { t } = useI18n();
+  const lp = useLocalePath();
   const [worldSetting, setWorldSetting] = useState("");
   const [styleGuide, setStyleGuide] = useState("");
   const [submitting, setSubmitting] = useState(false);
@@ -26,7 +28,7 @@ export function CustomForm() {
       JSON.stringify({ worldSetting, styleGuide }),
     );
     track("game_start", { source: "custom" });
-    router.push("/play?custom=1");
+    router.push(lp("/play?custom=1"));
   }
 
   return (