Line tool needs Ctrl to lock angle only after drag has begun

Ctrl is the shortcut for the Line tool to lock the current angle. But currently, it takes effect if Ctrl is held before the click and drag for drawing a new line, which is not desired, because Ctrl is also used to allow drawing a new line snapped to the endpoint of an existing line (rather than dragging around that endpoint). Ctrl should only lock the line's angle if it is pressed after the dragging is already occurring, whether that's for a new line or for dragging the endpoint of an existing line.

Start by locating the Line tool's pointer-drag handling and the logic for dragging an existing line endpoint. Trace how Ctrl is interpreted before and during a drag, then verify that starting with Ctrl preserves endpoint snapping while pressing it after dragging begins locks the angle in both cases.

Written by the indexing model from the issue text.