WS-4: React Flow full-rewrite of canvas

[espetro]

Plan: .agents/plans/2026-07-04-detach-canvas-pipeline.md (WS-4)

Replace custom div+CSS+iframe canvas with @xyflow/react.

• DesignIteration -> custom DesignFrameNode (iframe renderer)
• GenerationGroup -> ParentNode
• Replace CanvasArea.tsx viewport + use-canvas pan/zoom -> ReactFlow + useReactFlow + useViewport
• Rubber-band selection -> RF SelectionMode

FULL REWRITE (per decision 2026-07-05). Sequenced AFTER WS-1 (canvas detach) to avoid double-rewrite risk.

Effort: L (~3d). Depends on WS-1.