chore(deps): upgrade Astro 6.4 → 7.0 (Vite 8)

Bumps astro to 7.0.3 and @astrojs/check to the matching release. Vite 8
under the hood; @tailwindcss/vite/sitemap/rss already compatible, Node 22
satisfied. Verified end-to-end: typecheck (0 errors), build, dist scan,
audit gate, and visual render (hero + blog + Shiki) all clean under the new
Rust compiler + Markdown pipeline.

package.json

@@ -18,7 +18,7 @@
     "@astrojs/rss": "^4.0.18",
     "@astrojs/sitemap": "^3.7.3",
     "@tailwindcss/vite": "^4.3.1",
-    "astro": "^6.4.7",
+    "astro": "^7.0.3",
     "tailwindcss": "^4.3.1"
   },
   "devDependencies": {