Upgrade docs site to Astro 7 (clears remaining 5 audit findings)

After #220 and the follow-up npm audit fix (dff05dc), site/ has 5 remaining audit findings (2 high, 3 low). All of them require semver-major upgrades: [email protected], which cascades into @astrojs/starlight and @astrojs/mdx majors, plus [email protected].

The advisories are dev-server/SSR-context issues (XSS in define:vars, dev-server file read on Windows, sharp/libvips CVEs), so exposure for our statically built site is low. This is a planned migration rather than a quick patch: expect breaking-change review of starlight config and a full site build/visual check.

Acceptance:

• npm audit in site/ reports 0 high findings
• npm run build completes and the site renders correctly

Generated with Claude Code

https://claude.ai/code/session_01E6LA4sndtVGyvoXqwYeHoB