feat: Phase 1 - Canvas with React Flow

- 8 node types (Model, Solver, DesignVar, Extractor, Objective, Constraint, Algorithm, Surrogate)
- Drag-drop from palette to canvas
- Node configuration panels
- Graph validation with error/warning display
- Intent JSON serialization
- Zustand state management

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

atomizer-dashboard/frontend/src/components/canvas/nodes/ConstraintNode.tsx

@@ -0,0 +1,19 @@
+import { memo } from 'react';
+import { NodeProps } from 'reactflow';
+import { BaseNode } from './BaseNode';
+import { ConstraintNodeData } from '../../../lib/canvas/schema';
+
+function ConstraintNodeComponent(props: NodeProps<ConstraintNodeData>) {
+  const { data } = props;
+  return (
+    <BaseNode {...props} icon={<span>🚧</span>} color="text-orange-600">
+      {data.name && <div>{data.name}</div>}
+      {data.operator && data.value !== undefined && (
+        <div className="text-xs text-gray-400">
+          {data.operator} {data.value}
+        </div>
+      )}
+    </BaseNode>
+  );
+}
+export const ConstraintNode = memo(ConstraintNodeComponent);