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/app.go

@@ -245,6 +245,12 @@ func New(cfg *config.Config) *App {
 func (a *App) newGraphics() *kittyGraphics {
 	k := newKittyGraphics(a.writeTerminal)
 	k.cellPixels = func() (int, int) { return a.cellW, a.cellH }
+	k.background = func(prepare func() prepared, ready func(prepared)) {
+		go func() {
+			p := prepare()
+			a.post(func() { ready(p) })
+		}()
+	}
 	switch v := os.Getenv("ARR_TUI_DEBUG_GFX"); {
 	case v == "":
 	case strings.Contains(v, "/"):