feat(qbz-slint): route album artwork through the shared image cache

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

src-tauri/Cargo.lock

@@ -5074,6 +5074,8 @@ dependencies = [
  "dirs",
  "filetime",
  "log",
+ "md-5",
+ "rusqlite",
  "serde",
 ]