feat(slint): MPRIS album art (mpris:artUrl) — Qobuz pass-through + local file://

Port the Tauri normalizeCoverUrlForMetadata workaround as ArtworkRef::to_mpris_url
(qbz-models): remote Qobuz covers pass through as-is, local-file covers become a
percent-encoded file:// URI (MPRIS clients cannot read bare paths or asset://),
Plex thumbs become a tokenized HTTP URL. Wired into the Slint now-playing push so
the media widget shows album art. The app icon was already fixed via DesktopEntry.

cc

crates/qbz-models/Cargo.toml

@@ -21,3 +21,6 @@ thiserror = { workspace = true }
 
 # Time
 chrono = { workspace = true }
+
+# Correct file:// URI building for media-controls art (ArtworkRef::to_mpris_url)
+url = "2"