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/index.ts

@@ -0,0 +1,5 @@
+export { AtomizerCanvas } from './AtomizerCanvas';
+export { NodePalette } from './palette/NodePalette';
+export { NodeConfigPanel } from './panels/NodeConfigPanel';
+export { ValidationPanel } from './panels/ValidationPanel';
+export * from './nodes';