feat(website): add per-OS download landing pages

Add /download/ with Windows, macOS and Linux landing pages: OS-specific
install steps, requirements, feature list, FAQ, related posts and a
platform switcher, plus SoftwareApplication / FAQPage / BreadcrumbList
JSON-LD on every page.

Direct asset links resolve the latest published release from the GitHub
Releases API at build time (asset names, sizes, publish date) and fall
back to the root package.json version when the API is unreachable;
WEBSITE_SKIP_RELEASE_FETCH=1 forces the fallback. The deploy workflow
passes GITHUB_TOKEN to the build. The homepage download cards and the
header Download link now point at the new pages, the homepage schema
reads the resolved version instead of a hard-coded 0.20.0, and the
locale count is corrected to 19.

tools/testing/website-download-pages.test.mjs checks titles, canonicals,
direct asset links, structured data, cross-links and sitemap entries of
the built output; nx test website runs it beside the Giscus test.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

.github/workflows/deploy-website.yml

@@ -40,6 +40,9 @@ jobs:
 
       - name: Build website
         run: pnpm nx build website
+        env:
+          # Read by apps/website/src/lib/downloads.ts to resolve the latest published release.
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
 
       - name: Upload Pages artifact
         uses: actions/upload-pages-artifact@v5