deps(docs): bump astro and starlight ecosystem packages

- astro: ^5.18.0 → ^6.1.6 (fixes XSS in define:vars - CVE alerts #28, #29)
- @astrojs/sitemap: 3.7.0 → 3.7.1
- @astrojs/starlight: ^0.37.6 → ^0.39.1 (required for Astro 6)
- @astrojs/starlight-tailwind: ^4.0.2 → ^5.0.0
- starlight-image-zoom: ^0.13.2 → ^0.14.1
- starlight-versions: ^0.7.0 → ^0.8.1
- starlight-links-validator: ^0.19.2 → ^0.24.0 (required for Starlight >=0.38)
- starlight-llms-txt: ^0.7.0 → ^0.8.1 (required for Starlight >=0.38)
- starlight-scroll-to-top: ^0.4.0 → ^1.0.1 (required for Starlight >=0.38)
- starlight-sidebar-topics: ^0.6.2 → ^0.7.1 (required for Starlight >=0.38)
- starlight-tags: ^0.4.0 → ^1.0.1 (required for Starlight >=0.38)
- starlight-kbd: ^0.3.0 → ^0.4.0 (required for Starlight >=0.38)
- @tailwindcss/vite: ^4.2.1 → ~4.1.11 (pin to 4.1.x for Vite 7 compat)
- tailwindcss: ^4.2.1 → ~4.1.11 (pin to 4.1.x for Vite 7 compat)
- Removed zod override (3.25.76) — Astro 6 requires zod ^4.3.6

Author: AnExiledDev