v1.1.0 — Astro 6 upgrade

公式テーマカタログの鮮度対応として Astro 5 → 6 へアップグレードしました。Closes #25

Changed

• Upgraded to Astro 6 (astro@^6.0.0, @astrojs/mdx@^6.0.0) (#25)
• Updated @astrojs/rss to ^4.0.18 and @astrojs/sitemap to ^3.7.0
• Updated Three.js to ^0.184.0 (and @types/three)
• Updated @astrojs/check to ^0.9.9
• Migrated content collection schemas to import z from astro/zod (Zod 4)
• Bumped CI Node version to 22 and added engines.node >= 22

Full Changelog: v1.0.3...v1.1.0