feat(player): downscale + preload external cover art (8a)

AudioServiceConfig handed full-res album art to the notification /
lock screen / Wear. Add artDownscaleWidth/Height: 300 + preloadArtwork
so external surfaces get a smaller, faster, lower-memory cover with a
warm first paint.

6b (notification tap-to-open) dropped: androidNotificationClickStarts
Activity already defaults to true, so the tap foregrounds the app;
deep-linking to now-playing isn't a config knob and was judged
disproportionate.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

flutter_client/lib/main.dart

@@ -14,6 +14,13 @@ Future<void> main() async {
       androidNotificationChannelId: 'com.fabledsword.minstrel.audio',
       androidNotificationChannelName: 'Minstrel playback',
       androidNotificationOngoing: true,
+      // 8a: hand external surfaces (notification / lock screen / Wear)
+      // a downscaled cover instead of full-res album art — smaller
+      // payload, faster paint, lower memory. 300px is ample for those
+      // targets. preloadArtwork warms it so the first paint isn't blank.
+      artDownscaleWidth: 300,
+      artDownscaleHeight: 300,
+      preloadArtwork: true,
     ),
   );
   runApp(ProviderScope(