deps: add sharp as a direct dependency

Sharp is only an optional dep of astro; the CI runner omits optional deps,
so astro:assets image optimization failed with MissingSharp. Pinning it as
a direct dependency guarantees the native binary is installed.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

package.json

@@ -13,6 +13,7 @@
     "@astrojs/sitemap": "^3.3.0",
     "@tailwindcss/vite": "^4.1.5",
     "tailwindcss": "^4.1.5",
-    "@fontsource-variable/inter": "^5.2.5"
+    "@fontsource-variable/inter": "^5.2.5",
+    "sharp": "^0.34.5"
   }
 }