Real-image posters: scale and encode in the background

Done in Draw it took about 200 ms per new poster (BiLinear scale to the
box plus PNG encoding), so each step to another title stalled the UI.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/ui/kitty_test.go

@@ -417,3 +417,29 @@ func TestPostersAreSentAtTheBoxSize(t *testing.T) {
 		t.Errorf("upload without cell size: %q", msgs)
 	}
 }
+
+func TestPostersArePreparedInTheBackground(t *testing.T) {
+	var rec recorder
+	k := newKittyGraphics(rec.write)
+	var queued []func()
+	k.background = func(prepare func() prepared, ready func(prepared)) {
+		p := prepare()
+		queued = append(queued, func() { ready(p) })
+	}
+	img := image.NewRGBA(image.Rect(0, 0, 40, 60))
+	if id, err := k.show(img, img.Bounds(), 4, 3); id != 0 || err != nil || len(rec.cmds) != 0 {
+		t.Fatalf("first draw: id %d, err %v, %d commands", id, err, len(rec.cmds))
+	}
+	k.show(img, img.Bounds(), 4, 3) // a redraw meanwhile does not prepare it twice
+	if len(queued) != 1 {
+		t.Fatalf("%d preparations", len(queued))
+	}
+	queued[0]()
+	id, err := k.show(img, img.Bounds(), 4, 3)
+	if id == 0 || err != nil || len(rec.cmds) == 0 {
+		t.Fatalf("once ready: id %d, err %v, %d commands", id, err, len(rec.cmds))
+	}
+	if again, _ := k.show(img, img.Bounds(), 4, 3); again != id || len(queued) != 1 {
+		t.Errorf("uploaded posters are reused: id %d, %d preparations", again, len(queued))
+	}
+}