fix(playback): resume interrupted streams

[tofuliang]

Description

Preserve the current track and playback position when an MPV stream is interrupted by a network error.

Previously, MPV transitioning to Stopped cleared the current queue index. Pressing Play then restarted the first queue item from 0:00.

Changes

• Detect MPV END_FILE errors and premature EOF events.
• Add an optional player error callback.
• Preserve the current queue index and last observed position after an error.
• Resume the same track from that position when Play is pressed.
• Keep explicit Stop and normal end-of-track behavior unchanged.
• Report the interrupted state as paused.

Testing

• Added tests for error recovery, premature EOF, backward seeking, explicit Stop, queue preservation, and paused-state reporting.
• go test ./...
• go test -race ./backend/player/mpv ./backend
• Manually verified by interrupting a live TCP stream and restoring the connection.