Fix playlist track ordering to respect insertion order

- Add PlaylistLocalTrack struct with playlist_position field
- Add playlist_get_local_tracks_with_position command
- Update PlaylistDetailView to merge tracks by position instead of
  appending local tracks at the end
- Update PlaylistModal to calculate correct position when adding local
  tracks (total count = qobuz + existing local)
- Local tracks now appear in the order they were added, interleaved
  with Qobuz tracks

src-tauri/src/library/models.rs

@@ -104,6 +104,15 @@ impl Default for LocalTrack {
     }
 }
 
+/// A local track within a playlist, including its position
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct PlaylistLocalTrack {
+    #[serde(flatten)]
+    pub track: LocalTrack,
+    /// Position in the combined playlist (Qobuz + local tracks)
+    pub playlist_position: i32,
+}
+
 /// An album aggregated from local tracks
 #[derive(Debug, Clone, Serialize, Deserialize)]
 pub struct LocalAlbum {