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

packages/shared/src/links.test.ts

@@ -1,5 +1,5 @@
 import { describe, expect, it } from 'vitest';
-import { parseProfileLink, profileUrl } from './links';
+import { parseProfileLink, profileUrl, trackUrl } from './links';
 
 describe('profileUrl', () => {
   it('builds a website profile link, tolerating a trailing slash', () => {
@@ -34,3 +34,10 @@ describe('parseProfileLink', () => {
     expect(parseProfileLink(text)).toBeNull();
   });
 });
+
+describe('trackUrl', () => {
+  it('links to the song page, optionally saying who shared it', () => {
+    expect(trackUrl('https://m.test/', 'abc')).toBe('https://m.test/t/abc');
+    expect(trackUrl('https://m.test', 'abc', 'alice')).toBe('https://m.test/t/abc?from=alice');
+  });
+});

packages/shared/src/links.ts

@@ -29,6 +29,15 @@ export function parseProfileLink(text: string): string | null {
   return match ? valid(decodeURIComponent(match[1]!)) : null;
 }
 
+/** Shareable song page on the website, e.g. https://musicshare.app/t/<id>?from=alice */
+export function trackUrl(webBase: string, trackId: string, from?: string | null): string {
+  const url = `${webBase.replace(/\/+$/, '')}/t/${encodeURIComponent(trackId)}`;
+  return from ? `${url}?from=${encodeURIComponent(from)}` : url;
+}
+
+/** What goes in the share sheet next to the link. */
+export const shareText = (track: { title: string; artist: string }) => `${track.title} by ${track.artist}`;
+
 function valid(name: string): string | null {
   const username = name.toLowerCase();
   return USERNAME.test(username) ? username : null;