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- Delete 11 one-off migration scripts and outdated i18n docs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/i18n/config.ts

@@ -1,53 +1,12 @@
 // Supported locales for InfiPlot
 export const DEFAULT_LOCALE = "zh-CN" as const;
 
-export type Locale =
-  | "en"
-  | "zh-CN"
-  | "zh-TW"
-  | "zh-HK"
-  | "ja"
-  | "ko"
-  | "es"
-  | "fr"
-  | "de"
-  | "pt-BR"
-  | "pt"
-  | "ru"
-  | "it"
-  | "vi"
-  | "th"
-  | "id"
-  | "tr"
-  | "pl"
-  | "nl"
-  | "uk"
-  | "hi"
-  | "cs";
+export type Locale = "zh-CN" | "en" | "ja";
 
 export const LOCALE_NAMES: Record<Locale, string> = {
-  "en": "English",
   "zh-CN": "简体中文",
-  "zh-TW": "繁體中文（台灣）",
-  "zh-HK": "繁體中文（香港）",
+  "en": "English",
   "ja": "日本語",
-  "ko": "한국어",
-  "es": "Español",
-  "fr": "Français",
-  "de": "Deutsch",
-  "pt-BR": "Português (Brasil)",
-  "pt": "Português",
-  "ru": "Русский",
-  "it": "Italiano",
-  "vi": "Tiếng Việt",
-  "th": "ภาษาไทย",
-  "id": "Bahasa Indonesia",
-  "tr": "Türkçe",
-  "pl": "Polski",
-  "nl": "Nederlands",
-  "uk": "Українська",
-  "hi": "हिन्दी",
-  "cs": "Čeština",
 };
 
 export const LOCALES: Locale[] = Object.keys(LOCALE_NAMES) as Locale[];