Feature: Allow custom-node creation from an existing flowchart

[lambertaurelle]

The issue:

As flowcharts grow in complexity, manually recreating logic for reuse is time-consuming and error-prone. Currently, there is no way to "extract" a functional pattern from an existing flowchart into a reusable component without starting from scratch in the custom node editor.

Proposed Solution

Add a "Convert to Custom Node" workflow that allows designers to bundle existing logic instantly.

User Workflow:

1. Select: Use the Selection tool to highlight a group of nodes/edges in an existing flow.
2. Action: Right-click the selection and select "Create Custom Node from Selection."
3. Naming: A prompt appears to name the new custom node.
4. The current flow is saved and the selected nodes in the original flow are replaced by a single instance of the new custom node (maintaining connections).
5. The user is redirected to the custom node editor to fine-tune the new component.