Add Dagre re-flow action

### Description

A good addition to the assistant would be to add the Dagre algorithm to do reflow tasks instead of relying on the AI.

I have already tested this particular algorithm and it works well.

Integrating it into assistant as an action AND as a dropdown clickable menu item means the user can execute it with or without AI.

Demo

For reference, this was achieved in the Flow Building MCP tools however it is lacking context (like grid size and node widths etc). by doing this in the assistant it can be used with and without AI and will have additional context available for better reflow.

### Epic/Story

_No response_

Start by locating the assistant action integration and the dropdown menu entry, then review the existing Flow Building MCP implementation referenced in the issue. The work is done when Dagre reflow can be triggered both through the assistant and directly from the menu, using the available layout context such as grid size and node widths.

Written by the indexing model from the issue text.