Lemenzo Gallery v1.20 - large-library responsiveness and build-safety

- Fixes a compile blocker in the legacy album offset compatibility path.
- Large-library MediaStore/index/search executors now use Android background thread priority.
- Album summary refresh cancels stale full-library scans instead of queueing repeated scans.
- Search cancels stale queries immediately when the user types a newer query.
- Empty search no longer scans MediaStore.
- Photos/Favorites/Album/Trash page tasks are cancelled on reload/destroy to reduce wasted I/O.
- v1.19 keyset paging, bounded thumbnails, capped memory cache, MediaStore debounce and premium LemenzoOS UI are retained.