[REQUEST] Allow selecting multiple associated Assets for a Technique

Is your feature request related to a problem? Please describe.
When adding associated Assets to a Technique, the current UI appears to only allow selecting a single Asset via a radio button. That makes it hard to represent Techniques that apply to more than one Asset.

Describe the solution you'd like
Update the associated Assets selection UI for Techniques from a single-select radio button control to a multi-select "check all that apply" control. This would let users associate multiple Assets with a Technique in one edit flow.

Describe alternatives you've considered
Keeping the current single-select behavior forces users to choose only one Asset at a time. Another option would be a multi-select dropdown, but checkboxes would make the available choices and current selections more obvious.

Additional context
This would be a usability improvement for Technique editing and better reflects cases where a Technique is relevant to several Assets, not just one.

Start by locating the Technique editing UI for associated Assets and the current radio-button selection control. Trace how the selected Asset is stored and submitted, then verify that checkbox selections allow multiple Assets and persist together when editing a Technique.