WS-1: Detach canvas into packages/canvas (MIT, headless)

[espetro]

Plan: .agents/plans/2026-07-04-detach-canvas-pipeline.md (WS-1)

Sever the canvas into a standalone MIT package with no internal Calca deps.

• Move single coupling point: groupsAtom (apps/web/src/features/design/state/groups-atoms) -> packages/shared or canvas-local
• Create packages/canvas/ with CanvasArea, DesignCard, PipelineStatus, use-canvas, canvas-atoms, types
• Strip #/features/{design,context-toolbar,settings}/* imports -> props/children/context
• Export: CanvasProvider, DesignFrame, useCanvasViewport, useDesignFrames

Effort: M (~2d). Blocks WS-3, WS-4. Related #8