Add the frontend-agnostic contract for the source-aware playback context:
- qbz-models: new `source` module with `PlaybackSource` (Qobuz / OfflineCache
/ Local / Plex; parses the existing stringly-typed `source` field, defaults
to Qobuz) and `ArtworkRef` (Remote / LocalFile / PlexThumb / Embedded /
None). `QueueTrack::source_kind()` + `QueueTrack::artwork_ref()` derive both
without changing the wire format. `PlaybackSource::is_qobuz_streamable()`
seeds the Qobuz Connect queue gate (only Qobuz-sourced tracks are castable).
- qbz-slint artwork pipeline: `fetch_and_decode_ref(&ArtworkRef, ...)` resolves
cover bytes per origin — HTTP/Plex through the disk cache, local files read
directly, embedded bytes in place. This fixes the root cause of local/Plex
artwork never reaching the UI (the loader was HTTP-only). The URL-only
`fetch_and_decode` becomes a thin wrapper, so all existing card/row jobs are
unchanged. The now-playing loader now takes an `ArtworkRef` derived from the
queue track, so local-library/Plex covers reach the now-playing bar.
Groundwork for the Local Library Slint port (source-aware playback context
designed in from the start). qbz-models tests pass; qbz-slint cargo check green.
cc
vicrodh
827ba9d77a