feat(seo): add metadataBase and OpenGraph/Twitter metadata

Public launch prep — link previews on social/messaging apps were
falling back to bare URLs with no title/description. No branded OG
image exists yet (public/ only has create-next-app placeholders), so
this ships text-only; a real 1200x630 image is a follow-up.

src/app/layout.tsx

@@ -14,8 +14,21 @@ const inter = Inter({
 });
 
 export const metadata: Metadata = {
-  title: "PawFeed",
+  metadataBase: new URL(process.env.NEXT_PUBLIC_APP_URL ?? "https://pawfeed.org"),
+  title: { default: "PawFeed", template: "%s · PawFeed" },
   description: "The social platform where pets are the stars.",
+  openGraph: {
+    title: "PawFeed",
+    description: "The social platform where pets are the stars.",
+    url: "/",
+    siteName: "PawFeed",
+    type: "website",
+  },
+  twitter: {
+    card: "summary",
+    title: "PawFeed",
+    description: "The social platform where pets are the stars.",
+  },
 };
 
 export const viewport: Viewport = {