feat(youtube): queue media for background cache and fill fields when rea

- Add `youtube.mediaCache.maxHeight` config option (default 720p)
- Background mode creates text-only cards while cache downloads, queues media updates, fills audio/image fields once cached file is ready
- Announce cache download start and readiness via overlay/OSD notifications
- Skip mpv stream indexes when generating audio from a YouTube cached file

src/config/definitions/options-core.ts

@@ -130,6 +130,13 @@ export function buildCoreConfigOptionRegistry(
       defaultValue: defaultConfig.youtube.mediaCache.mode,
       description: 'How YouTube card audio/images are extracted.',
     },
+    {
+      path: 'youtube.mediaCache.maxHeight',
+      kind: 'number',
+      defaultValue: defaultConfig.youtube.mediaCache.maxHeight,
+      description:
+        'Maximum video height downloaded for the YouTube background media cache. Set to 0 for unlimited.',
+    },
     {
       path: 'controller.enabled',
       kind: 'boolean',