[Image Resizer] Option to scale images by long edge (feature request)

### Description of the new feature / enhancement

Add an option to Image Resizer that allows users to specify the target pixel size for the longest edge of an image (width or height). Instead of requiring separate width and height inputs, users would input a single value (e.g., 1200px), and the tool would proportionally scale each image (landscape or portrait), so its longest side matches this length, preserving aspect ratio automatically. This helps when batch processing images with mixed orientations, resulting in a consistent maximum dimension across all files.

### Scenario when this would be used?

Frequently, users need to resize folders of photos containing both portrait and landscape images. With current options, you must either process them separately or accept inconsistent results for short/long sides. With this feature, a single batch job can unify the maximum dimension, ideal for uploading to sites or venues with long-side restrictions, for consistent web publishing, or for creating archives where uniform maximum size is needed. This significantly streamlines workflows for power users and everyday users alike.

### Supporting information

Lightroom supports an option to resize photos by long edge when exporting, which is very useful.

Start by locating the Image Resizer implementation and its existing width and height controls. Trace how dimensions are applied during batch processing, then define the long-edge option for both portrait and landscape images; done means one target value preserves aspect ratio and makes the longest side match it across a mixed-orientation batch.

Written by the indexing model from the issue text.