Song progress bar, and poll right after a track ends

- Feed and profile cards show a progress bar with elapsed/total time, only
  when the source reports a real position (Plex, Jellyfin, the Android
  reader). It ticks locally between updates. Apple Music's guessed
  position is no longer reported.
- The Android reader advances MediaSession's stale position to now, stamps
  when it was read (the server corrects for upload delay), and reports
  seeks. The server pushes an update to watchers when a position jumps.
- When a polled source says where in the song someone is, the next poll is
  scheduled just after the song ends (never sooner than 5s), instead of
  waiting out the full interval.

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

packages/shared/src/devices.ts

@@ -49,6 +49,8 @@ export const devicePlaybackSchema = z
     state: z.enum(['playing', 'paused', 'stopped']),
     track: reportedTrack.nullish(),
     positionMs: z.number().int().min(0).nullish(),
+    /** Phone's clock (epoch ms) when positionMs was read; lets the server account for upload delay. */
+    positionAt: z.number().int().positive().nullish(),
   })
   .refine((v) => v.state === 'stopped' || v.track, { message: 'track is required unless stopped', path: ['track'] });