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/hooks/usePrefersReducedMotion.ts

@@ -2,12 +2,16 @@ import { useEffect, useState } from 'react'
 
 const QUERY = '(prefers-reduced-motion: reduce)'
 
-/** 사용자의 감소 모션 설정을 따라간다. 설정이 바뀌면 다시 렌더한다. */
+/**
+ * 사용자의 감소 모션 설정을 따라간다. 설정이 바뀌면 다시 렌더한다.
+ * 빌드 때 미리 그린 HTML과 첫 렌더가 같아야 하므로 처음엔 false로 시작하고 마운트 뒤 실제 값을 읽는다.
+ */
 export function usePrefersReducedMotion(): boolean {
-  const [reduced, setReduced] = useState(() => window.matchMedia(QUERY).matches)
+  const [reduced, setReduced] = useState(false)
 
   useEffect(() => {
     const media = window.matchMedia(QUERY)
+    setReduced(media.matches)
     const onChange = () => setReduced(media.matches)
     media.addEventListener('change', onChange)
     return () => media.removeEventListener('change', onChange)