refactor(workflow-dashboard): reduce cyclomatic complexity in editor (#449)

- Extract helpers in assignLayers (bfsLayers, processTarget, placeIsolatedNodes, maxLayerExcludingEnd) to reduce complexity from 26 → ≤15
- Extract isProtectedNode and isFirstConditionalSibling helpers in onBeforeDelete (20 → ≤15)
- Extract handleEscape and handleUndoRedo in handleKeyDown (23 → ≤15)
- Extract buildNodeMap, sortTransitions, buildStepEdges, pushStepEdges, assignTargetHandles in transIn (33 → ≤15)
- Extract validateRoleNodeEdges and hasEmptyConditionOnIfEdge in validateRoleNodes (22 → ≤15)
- Remove unused state parameter from Form component in add-node.tsx
- Add vitest + 19 tests covering all refactored functions

Co-Authored-By: Claude Sonnet 4 <noreply@anthropic.com>

packages/workflow-dashboard/package.json

@@ -31,8 +31,10 @@
     "@types/react": "^19.2.14",
     "@types/react-dom": "^19.2.3",
     "@vitejs/plugin-react": "^6.0.2",
+    "@vitest/ui": "^4.1.7",
     "tailwindcss": "^4.2.4",
     "typescript": "^5.8.3",
-    "vite": "^8.0.13"
+    "vite": "^8.0.13",
+    "vitest": "^4.1.7"
   }
 }