[Feature request] Support for multiple flow line colors

### Description

Flow line components currently only support a single color. It would be useful to have multiple color options to help distinguish between different types of flows on a busy canvas (e.g., focus order vs. reading order, or happy path vs. error path).

The current purple color also struggles with dark backgrounds behind it.

### Possible approaches

- A variable mode specific to flow lines, keeping the variant count low
- A color property on the component itself

### Additional idea

Consider adding an optional boolean property for a highlight or glow effect at the origin and/or destination of a flow line. This could help draw attention to key start and end points in a flow.