feat(site): prerender every locale and publish structured data, llms.txt and sitemaps

Crawlers received an empty client-rendered shell (107 characters of text);
most AI crawlers do not run JavaScript, so the product was invisible to them.

- Build renders each locale to static HTML (/ for Korean, /en/ … /vi/) with
  src/entry-server.tsx + scripts/prerender.mjs; the browser hydrates the same
  locale. The language menu links to those pages instead of switching in place.
- Per-locale head: title, description, canonical, hreflang (+ x-default /en/),
  Open Graph and X cards with a per-locale 1200x630 image.
- JSON-LD graph: Organization, WebSite, WebPage, SoftwareApplication (KRW
  offers, version, release date, download URL from the canonical sources),
  HowTo and FAQPage. No invented ratings.
- robots.txt (search, ai-input and ai-train allowed; /app, /api, invites
  excluded), sitemap.xml with language alternates, llms.txt and llms-full.txt
  generated from the same translations and canonical values.
- IndexNow key and a post-deploy ping (Bing, Naver, Yandex).
- A factual "at a glance" section and two FAQ answers (recognised languages,
  where data is stored) in all 10 languages.
- Icons, apple-touch-icon and web manifest; asset base is now absolute so
  sub-path pages load the same bundle.

Verified: 3.6k-7.7k characters of text per page, no hydration warnings,
no overflow at 320/1440 in six locales, axe 0, Lighthouse mobile SEO,
accessibility and best practices 100.

site/src/i18n/index.ts

@@ -1,4 +1,4 @@
-import { createContext, useContext, useState, useEffect, useCallback, type ReactNode } from 'react'
+import { createContext, useContext, type ReactNode } from 'react'
 import { createElement } from 'react'
 import { en } from './locales/en'
 import { ko } from './locales/ko'
@@ -129,9 +129,15 @@ export interface Translations {
     soonTitle: string
     soon: [TitledText, TitledText]
   }
+  /** 사실만 모은 요약. 검색·AI 답변이 그대로 인용할 수 있게 짧은 정의형 문장으로 쓴다. */
+  facts: {
+    title: string
+    /** {version}·{date}·{pro}·{proPlus} 는 release.ts·plan-catalog 정본에서 채운다. */
+    rows: [TitledText, TitledText, TitledText, TitledText, TitledText, TitledText, TitledText, TitledText]
+  }
   faq: {
     title: string
-    items: [FaqItem, FaqItem, FaqItem, FaqItem, FaqItem, FaqItem]
+    items: [FaqItem, FaqItem, FaqItem, FaqItem, FaqItem, FaqItem, FaqItem, FaqItem]
   }
   footer: {
     description: string
@@ -173,32 +179,24 @@ export const LOCALES: LocaleMeta[] = [
 ]
 
 // ── Translations map ───────────────────────────────────
-const translations: Record<Locale, Translations> = {
+export const translations: Record<Locale, Translations> = {
   en, ko, ja, zh, es, fr, de, pt, ru, vi,
 }
 
-// ── Storage ────────────────────────────────────────────
-const STORAGE_KEY = 'd3ro-locale'
+/** 기본 언어. 루트(`/`)가 이 언어이고, 나머지는 `/{code}/` 에 따로 미리 그려진다. */
+export const DEFAULT_LOCALE: Locale = 'ko'
 
-function isLocale(value: string): value is Locale {
-  return value in translations
+export function isLocale(value: string): value is Locale {
+  return Object.prototype.hasOwnProperty.call(translations, value)
 }
 
-function detectLocale(): Locale {
-  try {
-    const stored = localStorage.getItem(STORAGE_KEY)
-    if (stored && isLocale(stored)) return stored
-  } catch { /* 저장소 차단 환경 */ }
-
-  const browserLang = navigator.language.toLowerCase()
-  const match = LOCALES.find((l) => browserLang === l.code || browserLang.startsWith(l.code + '-'))
-  return match ? match.code : 'en'
+/** 언어별 페이지 경로. 검색 엔진이 언어마다 다른 주소를 색인하도록 언어를 주소로 나눈다. */
+export function localePath(locale: Locale): string {
+  return locale === DEFAULT_LOCALE ? '/' : `/${locale}/`
 }
 
-function persistLocale(locale: Locale): void {
-  try {
-    localStorage.setItem(STORAGE_KEY, locale)
-  } catch { /* 저장소 차단 환경 */ }
+export function localeMeta(locale: Locale): LocaleMeta {
+  return LOCALES.find((l) => l.code === locale) ?? LOCALES[0]
 }
 
 // ── Context ────────────────────────────────────────────
@@ -206,7 +204,6 @@ interface I18nContextValue {
   locale: Locale
   bcp47: string
   t: Translations
-  setLocale: (locale: Locale) => void
 }
 
 const I18nContext = createContext<I18nContextValue | null>(null)
@@ -219,26 +216,12 @@ export function useI18n(): I18nContextValue {
 
 // ── Provider ───────────────────────────────────────────
 interface I18nProviderProps {
+  /** 페이지 언어. 빌드 때 언어별 HTML을 미리 그리고, 브라우저는 같은 언어로 이어받는다. */
+  locale: Locale
   children: ReactNode
 }
 
-export function I18nProvider({ children }: I18nProviderProps) {
-  const [locale, setLocaleState] = useState<Locale>(detectLocale)
-
-  const setLocale = useCallback((next: Locale) => {
-    setLocaleState(next)
-    persistLocale(next)
-  }, [])
-
-  const t = translations[locale]
-  const bcp47 = LOCALES.find((l) => l.code === locale)?.bcp47 ?? 'en-US'
-
-  // 문서 언어와 메타데이터도 선택한 언어를 따른다.
-  useEffect(() => {
-    document.documentElement.lang = locale
-    document.title = t.meta.title
-    document.querySelector('meta[name="description"]')?.setAttribute('content', t.meta.description)
-  }, [locale, t])
-
-  return createElement(I18nContext.Provider, { value: { locale, bcp47, t, setLocale } }, children)
+export function I18nProvider({ locale, children }: I18nProviderProps) {
+  const value = { locale, bcp47: localeMeta(locale).bcp47, t: translations[locale] }
+  return createElement(I18nContext.Provider, { value }, children)
 }