chore(website): upgrade astro from v5 to v6 (#1339)

- astro: ^5.18.1 → ^6.4.8
- @astrojs/node: ^9.5.3 → ^10.1.4 (v10 requires astro ^6.3.0)
- @astrojs/react: ^4.4.2 → ^5.0.7

No code changes needed: the project has no content collections, no
ViewTransitions, no legacy experimental flags, and all Astro imports use
stable APIs that are unchanged in v6.


Upgrade guide: https://docs.astro.build/en/guides/upgrade-to/v6/

Co-authored-by: Felix Hennig <felix-agent@felixhennig.com>
Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: 3 people