chore(deps): upgrade to Astro 6.0.0 and @astrojs/mdx 5.0.0

Major version upgrade — Astro 6 includes Vite 7, Shiki 4, and
improved static build performance. No breaking changes affected
this project's static output configuration.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: puikinsh