Deezer client imports config_manager locally in __init__ and stores
it as self._config, unlike the other clients which import at module
level. The allow_fallback line was referencing the wrong name.
Each streaming source (Tidal, Qobuz, HiFi, Deezer) now has an "Allow
quality fallback" checkbox in Settings. When disabled, the source only
tries the exact quality selected — if unavailable, it skips and lets
the orchestrator try the next source. Default is ON (current behavior).