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/src/main.rs

@@ -2551,6 +2551,16 @@ fn main() -> Result<(), Box<dyn std::error::Error>> {
                         playback::play_tracks(runtime, weak, handle, tracks, idx);
                     });
                 }
+                ("playlist", "cache") => {
+                    if let Ok(pid) = id.parse::<u64>() {
+                        offline_cache::cache_playlist(
+                            runtime.clone(),
+                            weak.clone(),
+                            handle.clone(),
+                            pid,
+                        );
+                    }
+                }
                 ("playlist", "play-all") => {
                     let runtime = runtime.clone();
                     let weak = weak.clone();

crates/qbz-slint/src/offline_cache.rs

@@ -310,6 +310,32 @@ pub fn cache_album(
     });
 }
 
+/// Cache a whole playlist for offline playback: fetch its tracks, then batch.
+pub fn cache_playlist(
+    runtime: Runtime,
+    weak: slint::Weak<AppWindow>,
+    handle: tokio::runtime::Handle,
+    playlist_id: u64,
+) {
+    let inner = handle.clone();
+    handle.spawn(async move {
+        let pl = match runtime.core().get_playlist(playlist_id).await {
+            Ok(p) => p,
+            Err(e) => {
+                log::error!("[qbz-slint] cache playlist {playlist_id} failed: {e}");
+                crate::toast::error_weak(&weak, "Couldn't load that playlist");
+                return;
+            }
+        };
+        let tracks: Vec<qbz_models::Track> = pl.tracks.map(|c| c.items).unwrap_or_default();
+        if tracks.is_empty() {
+            crate::toast::error_weak(&weak, "This playlist has no playable tracks");
+            return;
+        }
+        cache_tracks(runtime, weak, inner, tracks);
+    });
+}
+
 /// Remove a track's offline copy (DB row + on-disk bundle/file + library row).
 pub fn remove_cached(
     _runtime: Runtime,

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.