harden deezer auth errors and mixed playlist preflight

internal/app/app_test.go

@@ -535,6 +535,16 @@ func TestRipAlbumRequiresDeezerARL(t *testing.T) {
 	}
 }
 
+func TestRipPlaylistMixedRequiresDeezerAuth(t *testing.T) {
+	d := config.DefaultConfigData()
+	m := &Main{Config: &config.Config{File: d, Session: d}}
+
+	err := m.ripPlaylistMixed(context.Background(), "mix1", "Mix", []PlaylistTrackRef{{Source: "deezer", ID: "1"}})
+	if err == nil || !strings.Contains(err.Error(), "deezer") {
+		t.Fatalf("expected deezer auth error, got %v", err)
+	}
+}
+
 func TestApplyQobuzArtistFiltersRepeats(t *testing.T) {
 	albums := []collectionAlbum{
 		{ID: "a1", Title: "Album X", BitDepth: 16, Sampling: 44.1, Explicit: false},