feat: complete v0.4.0 implementation - Reports, Charts, Comparison, Dark Mode

Backend (@backend-dev):
- ReportService with PDF/CSV generation (reportlab, pandas)
- Report API endpoints (POST, GET, DELETE, download with rate limiting)
- Professional PDF templates with branding and tables
- Storage management with auto-cleanup

Frontend (@frontend-dev):
- Recharts integration: CostBreakdown, TimeSeries, ComparisonBar
- Scenario comparison: multi-select, compare page with side-by-side layout
- Reports UI: generation form, list with status badges, download
- Dark/Light mode: ThemeProvider, toggle, CSS variables
- Responsive design for all components

QA (@qa-engineer):
- E2E testing setup with Playwright
- 100 test cases across 7 spec files
- Visual regression baselines
- CI/CD workflow configuration
- ES modules fixes

Documentation:
- Add todo.md with testing checklist and future roadmap
- Update kickoff prompt for v0.4.0

27 tasks completed, 100% v0.4.0 delivery

Closes: v0.4.0 milestone

frontend/e2e/visual-regression.spec.ts

@@ -10,8 +10,19 @@
  */
 
 import { test, expect } from '@playwright/test';
-import { navigateTo, waitForLoading, createTestScenario, cleanupTestScenario } from './utils/test-helpers';
+import { 
+  navigateTo, 
+  waitForLoading, 
+  createScenarioViaAPI, 
+  deleteScenarioViaAPI,
+  startScenarioViaAPI,
+  sendTestLogs,
+  generateTestScenarioName,
+  setDesktopViewport,
+  setMobileViewport,
+} from './utils/test-helpers';
 import { newScenarioData } from './fixtures/test-scenarios';
+import { testLogs } from './fixtures/test-logs';
 import path from 'path';
 import fs from 'fs';
 import { fileURLToPath } from 'url';