fix(server): collage drawScaled uses center-crop instead of stretch

internal/playlists/collage.go

@@ -176,16 +176,43 @@ func fallbackGlyph() image.Image {
 	return img
 }
 
-// drawScaled copies src into dst.Rect, scaling with simple nearest-neighbor.
-// stdlib lacks high-quality scaling; nearest-neighbor is fine for a
-// 600x600 output where each cell is 300x300 — most album covers are
-// already 300-1500 pixels and the visual loss is minor.
+// drawScaled copies src into dst.Rect using a center-cropped "cover" fit
+// (the same model as BoxFit.cover / object-fit: cover in the clients).
+// Non-square sources are scaled so the *smaller* destination dimension is
+// fully filled and the larger axis is center-cropped, preserving aspect
+// ratio. Without this, banner-shaped or LP-shaped album art stretches in
+// the cell -- the album-coherent system playlists (new_for_you,
+// first_listens) make the stretching disproportionately visible because
+// fewer unique covers contribute, so each warped cell is a quarter of
+// the collage rather than diluted.
+//
+// Scaling itself stays nearest-neighbor -- stdlib lacks high-quality
+// scaling and dependency cost is unjustified for this 600x600 output.
 func drawScaled(dst draw.Image, r image.Rectangle, src image.Image) {
 	srcBounds := src.Bounds()
+	srcW := srcBounds.Dx()
+	srcH := srcBounds.Dy()
+	if srcW <= 0 || srcH <= 0 {
+		return
+	}
+	dstW := r.Dx()
+	dstH := r.Dy()
+	// Cover-fit: the side of src that maps to dst at the larger scale
+	// fully fills its axis; the other axis is center-cropped.
+	scaleX := float64(dstW) / float64(srcW)
+	scaleY := float64(dstH) / float64(srcH)
+	scale := scaleX
+	if scaleY > scale {
+		scale = scaleY
+	}
+	cropW := float64(dstW) / scale
+	cropH := float64(dstH) / scale
+	cropOffX := float64(srcBounds.Min.X) + (float64(srcW)-cropW)/2
+	cropOffY := float64(srcBounds.Min.Y) + (float64(srcH)-cropH)/2
 	for y := r.Min.Y; y < r.Max.Y; y++ {
+		sy := int(cropOffY + float64(y-r.Min.Y)*cropH/float64(dstH))
 		for x := r.Min.X; x < r.Max.X; x++ {
-			sx := srcBounds.Min.X + (x-r.Min.X)*srcBounds.Dx()/r.Dx()
-			sy := srcBounds.Min.Y + (y-r.Min.Y)*srcBounds.Dy()/r.Dy()
+			sx := int(cropOffX + float64(x-r.Min.X)*cropW/float64(dstW))
 			dst.Set(x, y, src.At(sx, sy))
 		}
 	}