AD-801: Add workflow canvas MVP

[alph-cmky]

Summary

• Add the workflow canvas MVP with Start, Agent, Tool, Condition, Human Approval, and End node support.
• Add a node palette, selected-node inspector, DAG edge rendering, and save-state validation.
• Back canvas validation with workflow-core safety validation and render invalid DAG errors before save.
• Add focused Vitest coverage and update the AD-801 backlog checklist.

Testing

• TDD red check: pnpm test apps/web/src/features/workflows/workflow-canvas.test.ts initially failed because workflow-canvas.js did not exist.
• pnpm test apps/web/src/features/workflows/workflow-canvas.test.ts
• pnpm lint && pnpm format && pnpm typecheck && pnpm test && pnpm build

Stacked on #44.

Closes #20