Posters: always a 2:3 box

The poster box followed the image's shape, so images Radarr/Sonarr hand
out in another shape came out smaller than the others in a list. The box
is now always 2:3: an image within 12% of that fills it, trimmed at the
long sides (block characters too, which were stretched before); one
further off is shown whole, centred in the box.

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

internal/ui/cover_test.go

@@ -358,3 +358,48 @@ func TestCellsStyleIsFlat(t *testing.T) {
 		t.Errorf("got %q %v %v", ch, fg, bg)
 	}
 }
+
+func TestPostersShareTheTwoByThreeBox(t *testing.T) {
+	sim := tcell.NewSimulationScreen("")
+	if err := sim.Init(); err != nil {
+		t.Fatal(err)
+	}
+	defer sim.Fini()
+	sim.SetSize(40, 30)
+
+	// extent returns the columns and rows covered by poster cells (white).
+	extent := func(w, h int) (x0, y0, x1, y1 int) {
+		img := image.NewRGBA(image.Rect(0, 0, w, h))
+		for i := range img.Pix {
+			img.Pix[i] = 255
+		}
+		sim.Clear()
+		c := newCoverView(quadrantStyle, nil, func() float64 { return 2 })
+		c.setImage(img)
+		c.SetRect(0, 0, 40, 30)
+		c.Draw(sim)
+		x0, y0, x1, y1 = 99, 99, -1, -1
+		for y := 0; y < 30; y++ {
+			for x := 0; x < 40; x++ {
+				_, _, st, _ := sim.GetContent(x, y)
+				if _, bg, _ := st.Decompose(); bg == tcell.NewRGBColor(255, 255, 255) {
+					x0, y0, x1, y1 = min(x0, x), min(y0, y), max(x1, x), max(y1, y)
+				}
+			}
+		}
+		return
+	}
+	ax0, ay0, ax1, ay1 := extent(2000, 3000) // the usual shape
+	bx0, by0, bx1, by1 := extent(2100, 3000) // 5% wider: trimmed, same box
+	if ax0 != bx0 || ay0 != by0 || ax1 != bx1 || ay1 != by1 {
+		t.Errorf("2:3 at %d,%d-%d,%d but 0.7 at %d,%d-%d,%d", ax0, ay0, ax1, ay1, bx0, by0, bx1, by1)
+	}
+	if ay0 != 0 || ax1 != 39 {
+		t.Errorf("the box sits at the top right: %d,%d-%d,%d", ax0, ay0, ax1, ay1)
+	}
+	// A square image is shown whole: as wide as the box, shorter, centred.
+	sx0, sy0, sx1, sy1 := extent(1000, 1000)
+	if sx0 != ax0 || sx1 != ax1 || sy1-sy0 >= ay1-ay0 || sy0 <= ay0 || (sy0-ay0)-(ay1-sy1) > 1 || (ay1-sy1)-(sy0-ay0) > 1 {
+		t.Errorf("square at %d,%d-%d,%d in the box %d,%d-%d,%d", sx0, sy0, sx1, sy1, ax0, ay0, ax1, ay1)
+	}
+}