[Feature request] Add flow line variant with labels at the end

### Description

Currently, flow line labels are positioned in the middle of the line. It would be useful to have a variant (or property toggle) that places labels at the start or end of the flow line instead.

### Use case

End-positioned labels can be easier to read when flow lines overlap or when the annotation canvas is dense. They also pair better with certain layout patterns where the destination is the focal point.

首先检查 Figma 资产库中的流线组件，以及当前标签位置的表示方式。确定 variant 还是属性切换更适合现有的组件结构，然后验证标签是否可以出现在开头或结尾，同时保留当前的中间位置。

由索引模型根据 Issue 内容生成。