Flow builder: live preview for Screen nodes #1944

Problem

The Screen node config (title / description / fields / object-form — expanded in #1927) is authored blind: there's no way to see what the screen will look like to the end user. The Debug simulator, when paused at a screen, shows only paused — it doesn't render the form. Screen flows are user-facing wizards, so designing a form without seeing it is error-prone (field order, labels, types, object-form layout).

Proposal

A live preview that renders the screen as the end user would see it — title, description, the input fields (or the full object-form when objectName is set). Surface it in the node config panel and/or in the Debug simulator when paused at a screen. Reuse the runtime ScreenSpec renderer (and plugin-form/ObjectForm for object-form mode) so the preview matches runtime exactly.

Acceptance criteria

• Preview reflects title, description (with {var} interpolation), fields, and object-form mode (objectName/defaults).
• Updates live as the node config changes.
• Reuses the runtime screen/form renderer (not a separate reimplementation — avoids design↔runtime drift, cf. the simulator divergence fixed in fix(studio/flow): wire decision branches to edges, expand screen config, align simulator #1927).

Pointers

• The flow-runner renders the ScreenSpec at runtime (the engine's screen executor emits it) — reuse that renderer.
• inspectors/flow-node-config.ts — screen config group.
• plugin-form ObjectForm for object-form mode.
• previews/FlowSimulatorPanel.tsx — the paused-at-screen state where a preview would also fit.