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- Delete 11 one-off migration scripts and outdated i18n docs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

app/[locale]/new/page.tsx

@@ -0,0 +1,68 @@
+import Link from "next/link";
+import { CustomForm } from "@/components/CustomForm";
+import { localePath } from "@/lib/i18n/navigation";
+import { isValidLocale } from "@/lib/i18n/utils";
+import { DEFAULT_LOCALE, type Locale } from "@/lib/i18n/config";
+
+export default async function NewPage({
+  params,
+}: {
+  params: Promise<{ locale: string }>;
+}) {
+  const { locale: rawLocale } = await params;
+  const locale: Locale = isValidLocale(rawLocale) ? rawLocale : DEFAULT_LOCALE;
+  const lp = (path: string) => localePath(path, locale);
+
+  return (
+    <div className="min-h-screen flex flex-col">
+      <header className="px-6 md:px-16 pt-7 md:pt-10 flex items-center justify-between">
+        <Link
+          href={lp("/")}
+          className="text-[10px] smallcaps text-clay-700 hover:text-clay-900 transition-colors flex items-center gap-2"
+        >
+          <i className="fa-solid fa-arrow-left text-[9px]" />
+          InfiPlot
+        </Link>
+        <span className="text-[10px] smallcaps text-clay-500">
+          编 织 一 个 世 界
+        </span>
+      </header>
+
+      <section className="px-6 md:px-16 pt-20 md:pt-32 pb-20 md:pb-24 flex-1">
+        <div className="grid grid-cols-12 gap-8 md:gap-16 max-w-6xl">
+          <div className="col-span-12 md:col-span-4 animate-fade-in">
+            <p className="text-[10px] smallcaps text-clay-500 mb-6">
+              Ⅳ · 无 题
+            </p>
+            <h1 className="font-serif text-[44px] md:text-[64px] text-clay-900 leading-[0.96] mb-8">
+              写下
+              <br />
+              <em className="italic text-clay-600">两段</em>
+              <br />
+              文字。
+            </h1>
+            <div className="hairline w-12 mb-6" />
+            <p className="font-serif text-base text-clay-700 leading-[1.7]">
+              第一段，勾勒故事所在的世界。第二段，描述世界应是什么模样 —
+              它的介质、氛围、颗粒感。
+            </p>
+            <p className="font-serif italic text-sm text-clay-500 mt-5 leading-relaxed">
+              两栏皆可任意语言。越具体，回报越具体。
+            </p>
+          </div>
+          <div className="col-span-12 md:col-span-7 md:col-start-6">
+            <CustomForm />
+          </div>
+        </div>
+      </section>
+
+      <footer className="px-6 md:px-16 pb-8">
+        <div className="hairline-full w-full mb-4" />
+        <div className="flex items-center justify-between text-[10px] smallcaps text-clay-500">
+          <span>MMXXVI</span>
+          <span className="num">Ⅰ · Ⅳ</span>
+        </div>
+      </footer>
+    </div>
+  );
+}