[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.

Start by locating the flow line components described in the issue and reviewing how their current single purple color is defined. Clarify whether the intended scope is multiple color options, improved contrast, the optional highlight or glow effect, or a combination; done should be agreed support for the selected scope.

Written by the indexing model from the issue text.