feat(slint): wire track context-menu actions (favorite/radio/share/playlist/nav)

Connects the previously-inert track context-menu items now that the
backends exist:

  - Add to library -> add_favorite("track").
  - Go to album / Go to artist -> resolve the track (get_track) to its
    album / performer and navigate there.
  - Create radio -> Qobuz track radio (new get_radio_track core wrapper
    over the existing qbz-qobuz endpoint + playback::play_track_radio).
  - Share Qobuz link -> https://www.qobuz.com/track/{id} to clipboard
    (new arboard dep + share module).
  - Share Song.link -> Odesli (api.song.link) pageUrl resolution to
    clipboard.
  - Add to playlist -> a global picker modal (PlaylistPickerModal +
    PlaylistPickerState/Actions + playlist_picker controller) listing
    the user's playlists; picking one calls add_tracks_to_playlist.

Add to mixtape / Make available offline / Track info stay unwired
(no backend yet).

cc

crates/qbz-slint/ui/state.slint

@@ -10,6 +10,7 @@ export struct AlbumCardItem {
     id: string,
     title: string,
     artist: string,
+    artist-id: string,     // for the clickable artist name -> artist page
     genre: string,         // shown in the hover overlay
     year: string,          // shown in the hover overlay
     quality-tier: string,  // "hires" | "cd" | "" — drives the badge icon
@@ -712,6 +713,28 @@ export global QueueState {
     in property <bool> infinite-play: false;
 }
 
+// One row in the "Add to playlist" picker.
+export struct PlaylistPickItem {
+    id: string,
+    name: string,
+    tracks-line: string,   // "N tracks"
+}
+
+// "Add to playlist" picker — a global overlay opened from the track
+// context menu. Holds the pending track id while the user picks a
+// destination playlist. Populated by crate::playlist_picker.
+export global PlaylistPickerState {
+    in-out property <bool> open: false;
+    in property <string> track-id;
+    in property <[PlaylistPickItem]> playlists: [];
+    in property <bool> loading: false;
+}
+
+export global PlaylistPickerActions {
+    callback pick(string /* playlist id */);
+    callback close();
+}
+
 // A playlist result in search. Qobuz returns up to four cover URLs per
 // playlist; the card renders them as a count-based collage.
 export struct SearchPlaylistItem {