Share songs, with link previews

- "Share ↗" on every song (web and mobile): the phone's share sheet, or
  copy the link on desktop. Links go to a new song page, /t/<id>?from=you,
  with big artwork and open-in buttons for every service.
- Server-rendered previews (Open Graph / Twitter cards) for song pages and
  profiles: title, who shared it or what they're playing, and the album
  art, avatar or logo. The web server fetches from the API over the
  compose network (INTERNAL_API_URL).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MzFywujuH9TiuXbr9vjoX8

apps/web/src/app/layout.tsx

@@ -8,7 +8,11 @@ import './globals.css';
 const inter = Inter({ subsets: ['latin'], variable: '--font-inter', display: 'swap' });
 const spaceGrotesk = Space_Grotesk({ subsets: ['latin'], variable: '--font-space-grotesk', display: 'swap' });
 
+// Previews need absolute image URLs. In production the API lives at <site>/api, so its origin is the site's.
+const siteUrl = process.env.SITE_URL ?? new URL(process.env.NEXT_PUBLIC_API_URL ?? 'http://localhost:3737').origin;
+
 export const metadata: Metadata = {
+  metadataBase: new URL(siteUrl),
   title: 'musicshare',
   description: "See what your friends are listening to, and play it on your own service.",
   applicationName: 'musicshare',