[Bugfix:TAGrading] Remove panel selector modal drag

[AtCloudDiwash]

Why is this Change Important & Necessary?

Fixes #12839

The Panel Selector modal was draggable with no boundary
constraints. This caused two bugs:

1. Dragging and releasing the modal anywhere caused it to
   vanish immediately due to click event bubbling
2. Dragging outside the viewport made the modal inaccessible
   and it would reopen at the same off-screen position

There is no functional reason for the modal to be draggable.

What is the New Behavior?

The Panel Selector modal is no longer draggable. It opens
in its default centered position every time.

What steps should a reviewer take to reproduce or test the bug or new feature?

1. Go to the grading interface as any grading role
2. Click the panel mode button
3. Verify the modal opens in center position
4. Verify the modal cannot be dragged

Automated Testing & Documentation

No automated tests added. The existing Cypress tests for
the panel switcher cover the modal functionality.

Other information

No breaking changes. No migrations required.