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.

首先定位 Line 工具的指针拖动处理，以及拖动现有线段端点的逻辑。跟踪 Ctrl 在拖动前和拖动期间的解释方式，然后验证：在按住 Ctrl 的情况下开始拖动会保留端点吸附，而在拖动开始后再按下它会在两种情况下锁定角度。

由索引模型根据 Issue 内容生成。