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- Delete 11 one-off migration scripts and outdated i18n docs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/PresetCard.tsx

@@ -2,6 +2,7 @@
 
 import { useRouter } from "next/navigation";
 import type { Preset } from "@/lib/presets";
+import { useLocalePath } from "@/lib/i18n/hooks";
 
 export function PresetCard({
   preset,
@@ -11,9 +12,10 @@ export function PresetCard({
   ordinal: string;
 }) {
   const router = useRouter();
+  const lp = useLocalePath();
   return (
     <button
-      onClick={() => router.push(`/play?preset=${preset.id}`)}
+      onClick={() => router.push(lp(`/play?preset=${preset.id}`))}
       className="group block w-full py-10 md:py-12 border-t border-clay-900/10 hover:border-clay-900/35 transition-[border-color,padding] duration-500 text-left"
     >
       <div className="flex items-baseline gap-6 md:gap-10">