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/playwright.config.ts

@@ -0,0 +1,69 @@
+import { defineConfig, devices } from '@playwright/test';
+
+/**
+ * Playwright E2E Test Configuration
+ * 
+ * Run with: npm run test:e2e
+ * UI mode: npm run test:e2e:ui
+ */
+export default defineConfig({
+  testDir: './tests/e2e',
+  
+  // Run tests in parallel
+  fullyParallel: true,
+  
+  // Fail CI if test.only is left in code
+  forbidOnly: !!process.env.CI,
+  
+  // Retry on CI only
+  retries: process.env.CI ? 2 : 0,
+  
+  // Parallel workers
+  workers: process.env.CI ? 1 : undefined,
+  
+  // Reporter configuration
+  reporter: [
+    ['html', { outputFolder: 'playwright-report' }],
+    ['list'],
+  ],
+  
+  // Global settings
+  use: {
+    // Base URL for navigation
+    baseURL: 'http://localhost:3003',
+    
+    // Collect trace on first retry
+    trace: 'on-first-retry',
+    
+    // Screenshot on failure
+    screenshot: 'only-on-failure',
+    
+    // Video on failure
+    video: 'on-first-retry',
+  },
+
+  // Browser projects
+  projects: [
+    {
+      name: 'chromium',
+      use: { ...devices['Desktop Chrome'] },
+    },
+    // Uncomment to test on more browsers
+    // {
+    //   name: 'firefox',
+    //   use: { ...devices['Desktop Firefox'] },
+    // },
+    // {
+    //   name: 'webkit',
+    //   use: { ...devices['Desktop Safari'] },
+    // },
+  ],
+
+  // Start dev server before tests
+  webServer: {
+    command: 'npm run dev',
+    url: 'http://localhost:3003',
+    reuseExistingServer: !process.env.CI,
+    timeout: 120000,
+  },
+});