Border
Add an even border sized as a percentage of the short edge.
All processing runs in your browser. No file leaves this device.
Border width is a percentage of the short edge, so a mixed set of landscape and portrait frames comes out with visually consistent borders instead of ones that vary with orientation.
The border is applied after resizing. Adding it first would mean the border itself gets scaled along with the image, and the width you set would not be the width you get.