Show whiteboard laser only while click-and-hold is active

## Problem
The whiteboard laser is currently visible continuously.

## Requested behavior
Make the laser appear only while the user is actively clicking and holding, and hide it when the click/hold ends.

## Acceptance criteria
- The laser is not visible by default.
- Pressing and holding activates and displays the laser.
- Releasing the click hides the laser immediately.

Start by locating the whiteboard laser implementation and the click-and-hold handlers that control its visibility. Verify the behavior in the whiteboard: the laser is hidden by default, appears while the pointer is held, and hides immediately when the hold ends.

Written by the indexing model from the issue text.