feat(canvas): Studio Enhancement Phase 1 & 2 - v2.0 architecture and file structure

Phase 1 - Foundation:
- Add NodeConfigPanelV2 using useSpecStore for AtomizerSpec v2.0 mode
- Deprecate AtomizerCanvas and useCanvasStore with migration docs
- Add VITE_USE_LEGACY_CANVAS env var for emergency fallback
- Enhance NodePalette with collapse support, filtering, exports
- Add drag-drop support to SpecRenderer with default node data
- Setup test infrastructure (Vitest + Playwright configs)
- Add useSpecStore unit tests (15 tests)

Phase 2 - File Structure & Model:
- Create FileStructurePanel with tree view of study files
- Add ModelNodeV2 with collapsible file dependencies
- Add tabbed left sidebar (Components/Files tabs)
- Add GET /api/files/structure/{study_id} backend endpoint
- Auto-expand 1_setup folders in file tree
- Show model file introspection with solver type and expressions

Technical:
- All TypeScript checks pass
- All 15 unit tests pass
- Production build successful

atomizer-dashboard/frontend/src/components/canvas/nodes/index.ts

@@ -1,4 +1,5 @@
 import { ModelNode } from './ModelNode';
+import { ModelNodeV2 } from './ModelNodeV2';
 import { SolverNode } from './SolverNode';
 import { DesignVarNode } from './DesignVarNode';
 import { ExtractorNode } from './ExtractorNode';
@@ -9,6 +10,7 @@ import { SurrogateNode } from './SurrogateNode';
 
 export {
   ModelNode,
+  ModelNodeV2,
   SolverNode,
   DesignVarNode,
   ExtractorNode,
@@ -18,8 +20,12 @@ export {
   SurrogateNode,
 };
 
+// Use ModelNodeV2 by default for enhanced dependency display
+// Set USE_LEGACY_MODEL_NODE=true to use the original
+const useEnhancedModelNode = !import.meta.env.VITE_USE_LEGACY_MODEL_NODE;
+
 export const nodeTypes = {
-  model: ModelNode,
+  model: useEnhancedModelNode ? ModelNodeV2 : ModelNode,
   solver: SolverNode,
   designVar: DesignVarNode,
   extractor: ExtractorNode,