feat: playlist art fix + Play/Shuffle buttons in context header

- Fix playlist art: API returns images/images150/images300 arrays, not
  image_rectangle; update Rust model and Qt header to use images300 →
  images150 → images with fallback
- Add Play (▶) and Shuffle (⇄) buttons to the album/playlist header;
  Play starts from track 1, Shuffle enables shuffle mode and plays from
  a randomised position

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

rust/src/api/models.rs

@@ -183,7 +183,12 @@ pub struct PlaylistDto {
     pub duration: Option<i64>,
     pub description: Option<String>,
     pub owner: Option<PlaylistOwnerDto>,
-    pub image_rectangle: Option<Vec<String>>,
+    /// 4-cover mosaic at 300 px (preferred)
+    pub images300: Option<Vec<String>>,
+    /// 4-cover mosaic at 150 px (fallback)
+    pub images150: Option<Vec<String>>,
+    /// 4-cover mosaic at 50 px (last resort)
+    pub images: Option<Vec<String>>,
     pub tracks: Option<TracksWrapper>,
 }