Stabilisation quick fixes: orderly close, visible version and update band, chat pop-ups on, Windows shaders and hwdec, render logs

- Close: hide the window, dispose every player, thumbnail job and the
  sound engine, leave, then close normally; Linux runner _exit (the
  NVIDIA EGL exit handler crashed 10/10 closes, 0/41 now); Windows tears
  down inside the message loop, then TerminateProcess; watchdogs.
- Version under the Settings tab; an update band instead of a dot,
  never over a video; checks at start and every 4 hours.
- Chat pop-ups default to All again.
- Shader read-back compared as a list (old mpv joins with ','), so
  Windows stops turning shaders off; Windows hwdec auto-copy.
- Render diagnostics (`mpv: render` lines); RAM-only demuxer cache pin
  silences the file-cache warning; a volume readout test.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

apps/desktop/lib/src/widgets/update_banner.dart

@@ -3,7 +3,7 @@ import 'package:sync_relay_core/core.dart';
 
 import '../update/update_controller.dart';
 
-// "Update available" card (Home update indicator dialog). Kicks off the one
+// "Update available" card (Settings → Show update dialog). Kicks off the one
 // background check on first build; renders nothing until there is an
 // update to offer. Never shown over playback.
 class UpdateBanner extends StatefulWidget {