extract metadata merging logic from each page into a helper function

lib/helpers/metadata.ts

@@ -0,0 +1,29 @@
+import defaultMetadata from "../config/metadata";
+import type { Metadata } from "next";
+
+// helper function to deep merge a page's metadata into the default site metadata
+export const addMetadata = (metadata: Metadata): Metadata => {
+  return {
+    ...defaultMetadata,
+    ...metadata,
+    openGraph: {
+      ...defaultMetadata.openGraph,
+      title: metadata.title as string,
+      description: metadata.description as string,
+      url: metadata.alternates?.canonical as string,
+      ...metadata.openGraph,
+    },
+    twitter: {
+      ...defaultMetadata.twitter,
+      ...metadata.twitter,
+    },
+    alternates: {
+      ...defaultMetadata.alternates,
+      ...metadata.alternates,
+    },
+    other: {
+      ...defaultMetadata.other,
+      ...metadata.other,
+    },
+  };
+};