Replace per-card Isar tracking streams with a single subscription in
TrackerSectionScreen.
- Build a mediaId -> Track index at the screen level
- Pass resolved Track? into TrackerLibraryImageCard
- Convert TrackerLibraryImageCard to a plain StatelessWidget
- Remove StreamBuilder, ConsumerStatefulWidget, and keep-alive boilerplate
- Keep the same UI while reducing widget-level DB work
This follows the same parent-index pattern used elsewhere and makes
the tracker library view lighter and easier to maintain.