Support flow inputs + conditional evaluation for simulation

[JamesRobertWiseman]

OpenFlowKit should support flow-level inputs and let decision nodes evaluate conditions against them.

Example:

Inputs:
• type
• priority
• approved

Conditions:
• type == "request"
• priority == "high"
• approved == true

This would let users simulate flows with real input values and clearly see the actual path taken, instead of only viewing a static diagram.

The first implementation of this should stay small and focus on:

1.	flow-level inputs
2.	condition evaluation in decision nodes
3.	branch selection from the result
4.	run mode with path highlighting based of inputs

This feels like a natural next step for OpenFlowKit’s existing diagramming + DSL foundation, and it creates a solid base for more advanced features later.