qconnect: allow offline-cache to be cast (one consistent rule)

Three sites disagreed: model blocked OfflineCache, live gate allowed it, TS
sync filter blocked qobuz_download. Decision: ALLOW — an offline copy's queue
id is the real Qobuz track id. Flip the TS sync filter and fix the stale model
doc comment to match the live gate.

Also add Rust lock-in tests (offline_cache_is_castable / offline_cache_is_admitted)
and restore the missing $app/environment vitest mock the config already aliases,
so queuePlaybackService.test.ts can load.

cc

crates/qconnect-core/src/admission.rs

@@ -82,4 +82,9 @@ mod tests {
             );
         }
     }
+
+    #[test]
+    fn offline_cache_is_admitted() {
+        assert!(evaluate_remote_queue_admission(TrackOrigin::QobuzOfflineCache).accepted);
+    }
 }