fix(deps): upgrade astro 5.18 -> 6.4.2 (security advisories) (#17)

Clears the remaining astro Dependabot alerts (XSS in define:vars via
incomplete </script> sanitization; server-island encrypted-parameter
replay). The site has no integrations and uses the current content-layer
config (src/content.config.ts), so the 5 -> 6 major is a clean bump - all
4 pages build. CI runs Node 22 (astro 6 requires >=22.12).

Author: avifenesh