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

docker-compose.yml

@@ -93,6 +93,9 @@ services:
         # Optional donation link shown on the website, e.g. DONATION_URL=https://ko-fi.com/you
         NEXT_PUBLIC_DONATION_URL: ${DONATION_URL:-}
     image: musicshare-web:local
+    environment:
+      # Server-side rendering (link previews) talks to the API inside the compose network.
+      INTERNAL_API_URL: http://api:4000
     ports:
       - "${WEB_PORT:-3737}:3737"
     init: true