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/queue_test.go

@@ -126,6 +126,117 @@ func TestClear_ResetsSource(t *testing.T) {
 	}
 }
 
+// A queue built from one album stops being that album the moment a
+// track from somewhere else joins it, so every path that adds one
+// drops the source. Before this, SetQueue was the only writer and
+// Clear the only clearer, so "Playing from Abbey Road" outlived every
+// append — and, being persisted, every restart too.
+func TestAppendPathsDropSource(t *testing.T) {
+	t.Parallel()
+
+	album := Source{Type: "album", ID: 1, Label: "Abbey Road"}
+
+	tests := []struct {
+		name   string
+		append func(q *Queue, paths []string)
+	}{
+		{
+			name: "AddTrack",
+			append: func(q *Queue, paths []string) {
+				q.AddTrack(paths[5])
+			},
+		},
+		{
+			name: "AddTracks",
+			append: func(q *Queue, paths []string) {
+				q.AddTracks(paths[5:7])
+			},
+		},
+		{
+			name: "InsertNext",
+			append: func(q *Queue, paths []string) {
+				q.InsertNext(paths[5])
+			},
+		},
+		{
+			name: "InsertNextTracks",
+			append: func(q *Queue, paths []string) {
+				q.InsertNextTracks(paths[5:7])
+			},
+		},
+		{
+			name: "InsertTracksAt",
+			append: func(q *Queue, paths []string) {
+				q.InsertTracksAt(paths[5:7], 1)
+			},
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			q, db := setupTestQueue(t)
+			paths := seedAudioFiles(t, db, 8)
+
+			q.SetQueue(paths[:5], 0, false, album)
+
+			if got := q.GetState().Source; got != album {
+				t.Fatalf("source before append: got %+v, want %+v", got, album)
+			}
+
+			tt.append(q, paths)
+
+			if got := q.GetState().Source; got != (Source{}) {
+				t.Errorf(
+					"source after %s: got %+v, want zero value",
+					tt.name, got,
+				)
+			}
+		})
+	}
+}
+
+// Removing and reordering deliberately do not drop it: a queue with a
+// track taken out of it is still that album, and the link still goes
+// somewhere true.
+func TestRemoveAndMoveKeepSource(t *testing.T) {
+	t.Parallel()
+
+	album := Source{Type: "album", ID: 1, Label: "Abbey Road"}
+
+	t.Run("RemoveTrack", func(t *testing.T) {
+		t.Parallel()
+
+		q, db := setupTestQueue(t)
+		paths := seedAudioFiles(t, db, 5)
+
+		q.SetQueue(paths, 0, false, album)
+		q.RemoveTrack(3)
+
+		if got := q.GetState().Source; got != album {
+			t.Errorf("source after RemoveTrack: got %+v, want %+v", got, album)
+		}
+	})
+
+	t.Run("MoveQueueTracks", func(t *testing.T) {
+		t.Parallel()
+
+		q, db := setupTestQueue(t)
+		paths := seedAudioFiles(t, db, 5)
+
+		q.SetQueue(paths, 0, false, album)
+		q.MoveQueueTracks([]int{0}, 3)
+
+		if got := q.GetState().Source; got != album {
+			t.Errorf(
+				"source after MoveQueueTracks: got %+v, want %+v",
+				got, album,
+			)
+		}
+	})
+}
+
 func TestSetQueue_WithStartIndex(t *testing.T) {
 	t.Parallel()