When using the Apple Music controller, album artwork fails to display for streamed tracks that have not been added to the user's Library. AppleScript's `raw data of artwork 1 of current track` returns empty data for these tracks.
This change adds a fallback that searches the iTunes Search API using the track's title and artist, then selects the correct result by matching against the album name from the response fields.
- Added iTunes Search API fallback in `AppleMusicController` for artwork retrieval when AppleScript returns empty artwork data
- Search query uses title and artist only to avoid false matches; album is validated against structured response fields to select the correct result
- Artwork is cached per track to avoid redundant network requests
- Simplified the AppleScript by removing the ineffective Library playlist lookup fallback and collapsed the duplicate macOS 26 / pre-26 script branches
- Replaced `JSONSerialization` dictionary access with `Decodable` structs (`ITunesSearchResponse`, `ITunesTrack`) to match codebase conventions