feat(slint): offline button on every Qobuz track row + cache-whole-playlist

- TrackRow show-download now defaults to true: every Qobuz track row (album,
  artist, label, playlist, mix, search, favorites) carries the offline
  cloud/check button. Non-Qobuz surfaces (future Local Library / Plex) opt out
  by setting it false.
- Playlist header gains a "make offline" CircleAction -> ("playlist","cache")
  -> offline_cache::cache_playlist (get_playlist -> batch its tracks), mirroring
  the album-header action.

qbz-slint compiles clean.

cc

crates/qbz-slint/ui/playlist/PlaylistView.slint

@@ -375,6 +375,18 @@ export component PlaylistView inherits Rectangle {
                                 }
                             }
                         }
+                        // Make the whole playlist available offline.
+                        VerticalLayout {
+                            horizontal-stretch: 0;
+                            alignment: center;
+                            CircleAction {
+                                icon: @image-url("../assets/icons/cloud-download.svg");
+                                enabled: PlaylistState.tracks.length > 0;
+                                clicked => {
+                                    root.media-action("playlist", PlaylistState.id, "cache");
+                                }
+                            }
+                        }
                         // Edit (rename / delete) — only the user's own.
                         if PlaylistState.is-owner: VerticalLayout {
                             horizontal-stretch: 0;

crates/qbz-slint/ui/primitives/TrackRow.slint

@@ -32,7 +32,10 @@ export component TrackRow inherits Rectangle {
     in property <bool> show-artwork: true;
     in property <bool> show-album: false;
     in property <bool> show-favorite: false;
-    in property <bool> show-download: false;
+    // On by default: every Qobuz track row carries the offline cloud/check
+    // button. A non-Qobuz surface (future Local Library / Plex rows, which are
+    // already local) should set this false.
+    in property <bool> show-download: true;
     in property <bool> multi-select-mode: false;
     // Custom-order reorder controls (up/down chevrons) at the leading
     // edge — used by the playlist custom sort.