Bulk remove image assets from asset folders

Users can import large batches of project image assets, but deleting them currently requires removing each asset one at a time. Add bulk removal in the image assets area so users can quickly remove every image in a selected asset folder, or all image assets in a project, without manually deleting hundreds of entries.

The motivating report is a project image folder with 512 pictures where deleting around 100 images one by one takes too much time. The accepted first scope is folder/root bulk removal. True multi-selection in the Navigator can be considered later if folder-level actions do not cover enough of the workflow.