chore(deps): upgrade astro from 5.16.8 to 6.4.8

First of two upgrade steps (5→6, then 6→7). The codebase was audited
for HTML validity and whitespace dependencies before this bump:
no <div> inside <p>, no <a> inside <a>, no adjacent inline elements
without explicit space.

Astro 6 brings Vite 7, Zod 4, Shiki 4 and Node 22+ requirement.
This project already runs on Node 24 (.nvmrc) and doesn't use content
collections, Shiki APIs, or server adapters, so the impact is minimal.

Verified: astro check passes 0/0/0 (was 0/0/2 hints in v5), dev server
returns 200 on /es/ with social icons, view transitions and i18n
hreflang tags all rendering correctly.

Author: francescarpi

package.json

@@ -22,7 +22,7 @@
     "@lucide/astro": "1.21.0",
     "@tailwindcss/vite": "4.3.1",
     "animejs": "4.5.0",
-    "astro": "5.16.8",
+    "astro": "6.4.8",
     "tailwindcss": "4.3.1"
   },
   "devDependencies": {