Support flow control (backpressure) visualization in frontend

### Feature Summary

The frontend should make flow control behavior directly visible on the workflow graph so users can immediately understand where execution pressure is building up during runtime. Color and visual emphasis should play an important role in communication.

### Proposed Solution or Design

At a high level, this feature can be implemented as a visualization layer on top of the existing workflow graph. The frontend would consume runtime flow control information provided by the backend and render it directly on workflow edges so that users can see where congestion is emerging as execution progresses.

The visualization should combine both color and compact edge labels. Color can be used to communicate the severity of congestion at a glance, while the label can display the current flow-control usage in a more precise form, including the current size and the current percentage of utilization. This gives users both a quick visual signal and concrete runtime information for inspection.

### Impact / Priority

(P2)Medium – useful enhancement

### Affected Area

_No response_