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-slint/Cargo.toml

@@ -23,6 +23,7 @@ open = "5"
 qbz-app = { path = "../qbz-app" }
 qbz-core = { path = "../qbz-core" }
 qbz-models = { path = "../qbz-models" }
+qbz-cache = { path = "../qbz-cache" }
 
 [build-dependencies]
 slint-build = "1"