Fix proportional scaling when single dimension is 0

When only width or height is specified (the other being 0), scale the
image proportionally to maintain aspect ratio. Previously, 0 was passed
directly to the resize function which produced a 0x0 image.

internal/imgcache/processor.go

@@ -66,10 +66,17 @@ func (p *ImageProcessor) Process(
 	targetWidth := req.Size.Width
 	targetHeight := req.Size.Height
 
-	// If both are 0, keep original size
+	// Handle dimension calculation
 	if targetWidth == 0 && targetHeight == 0 {
+		// Both are 0: keep original size
 		targetWidth = origWidth
 		targetHeight = origHeight
+	} else if targetWidth == 0 {
+		// Only height specified: calculate width proportionally
+		targetWidth = origWidth * targetHeight / origHeight
+	} else if targetHeight == 0 {
+		// Only width specified: calculate height proportionally
+		targetHeight = origHeight * targetWidth / origWidth
 	}
 
 	// Resize if needed