[Improvement] Add min/max width constraints to Flow Lines

### Description

Flow line components currently have no minimum or maximum width constraints. On a busy canvas, flow lines can end up very narrow (hard to read) or very wide (taking up too much space).

### Requested change

Add optional min and/or max width constraints to flow line components so they stay within a usable size range.

Start by locating the flow line components in the Figma asset library and inspecting their existing sizing properties. Add optional minimum and maximum width constraints, then verify on a busy canvas that flow lines stay within the configured usable range.

Written by the indexing model from the issue text.