Cover art now goes through the QBZ image cache instead of being
downloaded directly every time, matching the Tauri behavior.
- Moves ImageCacheService / ImageCacheStats into the qbz-cache crate
(LRU disk cache, MD5-keyed, SQLite index). src-tauri/image_cache.rs
re-exports it and keeps the Tauri managed-state wrapper, so the Tauri
app stays behavior-identical and both shells share one implementation
and one on-disk cache (~/.cache/qbz/images).
- qbz-slint artwork.rs checks the cache first, downloads and stores on
a miss, and trims the cache to 200MB on startup.
cc
vicrodh
3e61ec3b0f