feat(slint): playlist detail view

First piece of the playlist module: a detail view at the AlbumView /
MixView level, reusing the shared TrackRow (show-artwork + show-album +
context menu, so Add to playlist / Go to album / etc. work in it).

  - PlaylistState + crate::playlist controller (get_playlist -> header
    metadata + TrackItem rows; cached track list backs play-all /
    per-track play; artwork jobs resolve row covers + the header cover).
  - PlaylistView.slint: cover + "Playlist" label + name + description +
    "owner - N tracks - duration" info line + circular actions (Play
    all, Shuffle, Favorite) over a column-headed track list.
  - ContentView.playlist + NavEntry::Playlist (back/forward history) +
    navigate_playlist; the Favorites > Playlists card now opens it.
  - media-action: playlist play-all / shuffle / favorite (add/remove
    favorite) + per-track play-from-here.

cc

crates/qbz-slint/ui/state.slint

@@ -196,7 +196,23 @@ export global AlbumActions {
 }
 
 // Which content view the shell shows.
-export enum ContentView { home, album, artist, settings, search, musician, label, location, favorites, mix }
+export enum ContentView { home, album, artist, settings, search, musician, label, location, favorites, mix, playlist }
+
+// Playlist detail screen state. Rows reuse the shared TrackRow
+// (show-artwork + show-album); populated by crate::playlist.
+export global PlaylistState {
+    in property <string> id;
+    in property <string> name;
+    in property <string> owner;
+    in property <string> description;
+    in property <int> track-count: 0;
+    in property <string> total-duration;
+    in property <string> cover-url;
+    in property <image> cover;
+    in property <bool> is-favorite: false;
+    in property <[TrackItem]> tracks: [];
+    in property <bool> loading: false;
+}
 
 // Settings screen state. Audio and Playback preferences are loaded from
 // the persistence stores by the Rust settings controller and pushed here;