fix(playback): stop external player position polling that starts after exit (#1720)

MPV and VLC wait 2 s and 1.5 s before their first position poll, but the
delay timer was never stored, so stopping the poll during that wait could
not cancel it. A player that exited, errored or was replaced early left a
5 s (MPV) or 2 s (VLC) interval polling a dead socket or RC port until the
next launch or app quit. Keep the delay handle and clear it with the
interval.

Co-authored-by: 4gray <fourgray@proton.me>
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

.changes/playback-external-poll-after-exit.md

@@ -0,0 +1,7 @@
+---
+type: fix
+area: playback
+---
+
+Closing MPV or VLC within the first seconds of playback no longer leaves the
+app checking the closed player's position in the background until it quits.