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/vitest.config.ts

@@ -0,0 +1,31 @@
+import { defineConfig } from 'vitest/config';
+import react from '@vitejs/plugin-react';
+
+export default defineConfig({
+  plugins: [react()],
+  test: {
+    environment: 'jsdom',
+    globals: true,
+    setupFiles: ['./src/test/setup.ts'],
+    include: ['src/**/*.{test,spec}.{ts,tsx}'],
+    exclude: ['node_modules', 'dist', 'tests/e2e'],
+    coverage: {
+      provider: 'v8',
+      reporter: ['text', 'html', 'lcov'],
+      include: ['src/**/*.{ts,tsx}'],
+      exclude: [
+        'src/test/**',
+        'src/**/*.d.ts',
+        'src/vite-env.d.ts',
+        'src/main.tsx',
+      ],
+    },
+    // Mock CSS imports
+    css: false,
+  },
+  resolve: {
+    alias: {
+      '@': '/src',
+    },
+  },
+});