fix(og-images): wrap Buffer in Uint8Array for Response body (#568)

Astro uses Web API typings; Response doesn’t accept Node Buffer. Convert
the generated OG image Buffer to Uint8Array before constructing Response
to satisfy BodyInit and fix the build/type error.

Closes #560

src/pages/posts/[...slug]/index.png.ts

@@ -27,10 +27,8 @@ export const GET: APIRoute = async ({ props }) => {
     });
   }
 
-  return new Response(
-    await generateOgImageForPost(props as CollectionEntry<"blog">),
-    {
-      headers: { "Content-Type": "image/png" },
-    }
-  );
+  const buffer = await generateOgImageForPost(props as CollectionEntry<"blog">);
+  return new Response(new Uint8Array(buffer), {
+    headers: { "Content-Type": "image/png" },
+  });
 };