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

crates/qbz-cache/Cargo.toml

@@ -15,3 +15,7 @@ serde = { version = "1", features = ["derive"] }
 # File system utilities
 dirs = "6"
 filetime = "0.2"
+
+# Image cache index + URL hashing
+rusqlite = { version = "0.31", features = ["bundled"] }
+md-5 = "0.10"