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.

.forgejo/workflows/deploy-site.yml

@@ -84,3 +84,12 @@ jobs:
           curl --fail --silent --show-error https://d3ro.chanpaca.net/privacy/ --output /dev/null
           curl --fail --silent --show-error https://d3ro.chanpaca.net/terms/ --output /dev/null
           curl --fail --silent --show-error https://d3ro.chanpaca.net/delete-account/ --output /dev/null
+          # 검색·AI 검색용 산출물(언어별 미리 그린 페이지, sitemap, robots, llms.txt)
+          for path in en/ ja/ sitemap.xml robots.txt llms.txt llms-full.txt; do
+            curl --fail --silent --show-error "https://d3ro.chanpaca.net/$path" --output /dev/null
+          done
+          curl --fail --silent --show-error https://d3ro.chanpaca.net/en/ | grep -q 'application/ld+json'
+
+      - name: IndexNow 알림 (Bing·네이버·Yandex)
+        continue-on-error: true
+        run: node scripts/ci/ping-indexnow.mjs