feat: add Microsoft Store downloads and unified store cards

[pcodedynamics]

Funput is now available on Microsoft Store. Make the Store the primary Windows download destination and give Windows, iOS and Android equal space in the platforms section with their own store cards.

Changes:

• Add shared store buttons and responsive cards for Microsoft Store, App Store and Google Play, with macOS and Linux download cards below.
• Update the Windows page, homepage announcement, footer and installation articles with Store installation and update guidance; retain portable Windows downloads.
• Align Windows structured data, llms.txt, README and SEO checks with the public Microsoft Store listing.

Validation:

• pnpm format:check
• pnpm check — no errors, warnings or hints.
• pnpm build — 19 pages built.
• pnpm seo:check — 19 HTML pages and 18 sitemap URLs checked.
• git diff --check
• Browser checks at desktop, tablet and mobile widths, including 320px without horizontal overflow; verified store destinations.