[Parley] FlowView: Shift+drag panning broken in side-by-side mode, scrollbars missing

Description

In side-by-side mode (TreeView + FlowView), the FlowView panel has issues:

1. Shift+drag panning doesn't work - Cannot pan the flowchart using Shift+left-click drag
2. Scrollbars are missing - No scrollbars visible to navigate large flowcharts

Expected Behavior

• Shift+left-click should allow panning the flowchart view
• Scrollbars should appear when flowchart content exceeds viewport

Steps to Reproduce

1. Open a dialog file
2. Enable side-by-side mode (TreeView + FlowView)
3. Try Shift+drag on the flowchart - doesn't pan
4. Notice scrollbars are not visible

Context

Part of Sprint #451 testing

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