fix: add id field to PWA manifest (#570)

without an explicit id, iOS may conflate different PWAs when routing
media session events. this caused tapping track metadata in control
center to open the wrong home screen app (leaflet.pub instead of plyr.fm).

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Opus 4.5 <[e-mail]>

frontend/static/manifest.webmanifest

···
 {
   "name": "plyr.fm",
+  "id": "fm.plyr",
   "short_name": "Plyr",
   "start_url": "/",
   "display": "standalone",