fix(qbz-qt): playlist artwork — own graphic for Qobuz, collage for the rest

Two bugs with one root: the card bound the playlist's `images*` lists, which
are MEMBER-ALBUM covers, not the playlist's own artwork. So a Qobuz editorial
playlist showed one of its albums' sleeves, and a user playlist — which has no
`images*` at all — showed nothing.

Qobuz playlists carry their graphic in `image_rectangle`, which the model did
not deserialize; the field is added (additive, `serde(default)`, and Playlist
is only ever built through serde) and the card prefers it, contain-fitted so
the artwork is not cropped. Playlists without one fall back to the member-cover
mosaic, reusing the collage the sidebar already draws rather than a second
implementation. Both the All-tab grid and its list mode are covered — the 44px
list thumb binds the same map and would otherwise have gone blank.

cc

crates/qbz-qt/qml/views/LibraryView.qml

@@ -356,9 +356,24 @@ Rectangle {
                 clip: true
                 RoundedImage {
                     anchors.fill: parent
+                    visible: !lrCollage.visible
                     source: root.artMap[item.artKey] || ""
                     radius: item.kind === "artist" ? 22 : 6
-                    fit: item.kind === "label" ? "contain" : "crop"
+                    // A Qobuz playlist's own graphic is contain-fitted like a
+                    // label logo — cropping it cuts the wordmark.
+                    fit: (item.kind === "label" || item.playlistOwnImage === true)
+                        ? "contain" : "crop"
+                }
+                // User playlists have no graphic of their own, so they show the
+                // member-cover mosaic instead of a blank tile.
+                PlaylistCollage {
+                    id: lrCollage
+                    anchors.fill: parent
+                    visible: item.kind === "playlist"
+                        && item.playlistOwnImage !== true
+                        && (root.artMap[item.artKey] || "") === ""
+                    urls: item.covers || []
+                    radius: 6
                 }
                 Rectangle {
                     visible: item.kind === "track" || item.kind === "album" || item.kind === "playlist"