fix(queue): stop claiming a queue came from somewhere it no longer does

`q.source` was written by SetQueue and cleared in exactly one place,
Clear, so no append path touched it: adding a track to a queue built
from an album left the page still offering "Playing from <that album>",
and since the source is persisted alongside the queue state the wrong
label outlived the session that earned it.

Every add and insert path drops it now. Removing and reordering
deliberately do not — a queue with a track taken out of it is still
that album, and the link still goes somewhere true. Only the arrival of
a track from elsewhere makes the claim false.

The delta event carries the source for the same reason it carries the
current index: an append emits nothing else, so the frontend would keep
the label it was last given until something forced a full state.

Closes #14

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

backend/queue/emit_test.go

@@ -219,6 +219,56 @@ func TestEmit_AddTrackSendsDeltaNotSnapshot(t *testing.T) {
 	}
 }
 
+// The append clears the source, and the delta is the only event those
+// paths emit — so if it does not carry the source, the frontend keeps
+// the label it was last given and goes on offering a link back to an
+// album the queue no longer holds until something forces a full state.
+func TestEmit_AppendDeltaCarriesClearedSource(t *testing.T) {
+	t.Parallel()
+
+	q, db, rec := setupRecordedQueue(t)
+	paths := seedAudioFiles(t, db, 4)
+
+	q.SetQueue(
+		paths[:3], 0, false,
+		Source{Type: "album", ID: 1, Label: "Abbey Road"},
+	)
+
+	if _, ok := rec.Wait(events.QueueChanged, waitFor); !ok {
+		t.Fatalf("no QueueChanged after SetQueue; got %v", rec.Names())
+	}
+
+	rec.Reset()
+	q.AddTrack(paths[3])
+
+	if got := modifiedOf(t, rec).Source; got != (Source{}) {
+		t.Errorf("delta source = %+v, want zero value", got)
+	}
+}
+
+// And a delta that did not clear it still reports the source it has,
+// or the frontend would drop a perfectly good label on every removal.
+func TestEmit_NonAppendDeltaCarriesSource(t *testing.T) {
+	t.Parallel()
+
+	q, db, rec := setupRecordedQueue(t)
+	paths := seedAudioFiles(t, db, 4)
+
+	album := Source{Type: "album", ID: 1, Label: "Abbey Road"}
+	q.SetQueue(paths, 0, false, album)
+
+	if _, ok := rec.Wait(events.QueueChanged, waitFor); !ok {
+		t.Fatalf("no QueueChanged after SetQueue; got %v", rec.Names())
+	}
+
+	rec.Reset()
+	q.RemoveTrack(3)
+
+	if got := modifiedOf(t, rec).Source; got != album {
+		t.Errorf("delta source = %+v, want %+v", got, album)
+	}
+}
+
 func TestEmit_RemoveTracksReportsPositions(t *testing.T) {
 	t.Parallel()