[enhancement]: QoL Image gallery - Add thumbnail checkbox for multi-selection/keyboard toggle

### Is there an existing issue for this?

- [X] I have searched the existing issues

### Contact Details

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### What should this feature add?

Multi-selection via mouse (holding CTRL to tag multiple and SHIFT for a from-to range) is a nice first step, but prone to unwanted behaviour (e.g. accidentally clicking without holding down CTRL will deselect all previously selected images). On the other hand, it's a quick and established way to deselect all items, so messing with that mechanic does not seem ideal.

Adding an (optional?) checkbox in the lower right corner of the thumbnail - similar to the optional trashcan icon - would be a QoL feature in my opinion fixing the problem. Allowing selecting multiple images with just using the mouse.
Adding a keyboard shortcut (INS key, for instance) to toggle the selection status of the current image would be another nice thing to have, allowing you to quickly go through the gallery and tag files for deletion or moving to a board.

### Alternatives

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### Aditional Content

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