feat: implement v0.4.0 - Reports, Charts, Comparison, Dark Mode, E2E Testing

Backend (@backend-dev):
- Add ReportService with PDF/CSV generation (reportlab, pandas)
- Implement Report API endpoints (POST, GET, DELETE, download)
- Add ReportRepository and schemas
- Configure storage with auto-cleanup (30 days)
- Rate limiting: 10 downloads/minute
- Professional PDF templates with charts support

Frontend (@frontend-dev):
- Integrate Recharts for data visualization
- Add CostBreakdown, TimeSeries, ComparisonBar charts
- Implement scenario comparison page with multi-select
- Add dark/light mode toggle with ThemeProvider
- Create Reports page with generation form and list
- Add new UI components: checkbox, dialog, tabs, label, skeleton
- Implement useComparison and useReports hooks

QA (@qa-engineer):
- Setup Playwright E2E testing framework
- Create 7 test spec files with 94 test cases
- Add visual regression testing with baselines
- Configure multi-browser testing (Chrome, Firefox, WebKit)
- Add mobile responsive tests
- Create test fixtures and helpers
- Setup GitHub Actions CI workflow

Documentation (@spec-architect):
- Create detailed kanban-v0.4.0.md with 27 tasks
- Update progress.md with v0.4.0 tracking
- Create v0.4.0 planning prompt

Features:
✅ PDF/CSV Report Generation
✅ Interactive Charts (Pie, Area, Bar)
✅ Scenario Comparison (2-4 scenarios)
✅ Dark/Light Mode Toggle
✅ E2E Test Suite (94 tests)

Dependencies added:
- Backend: reportlab, pandas, slowapi
- Frontend: recharts, date-fns, @radix-ui/react-checkbox/dialog/tabs
- Testing: @playwright/test

27 tasks completed, 100% v0.4.0 implementation

frontend/e2e/screenshots/baseline/README.md

@@ -0,0 +1,30 @@
+# Baseline Screenshots
+
+This directory contains baseline screenshots for visual regression testing.
+
+## How to add baselines:
+
+1. Run tests to generate initial screenshots
+2. Review the screenshots in `e2e/screenshots/actual/`
+3. Copy approved screenshots to this directory:
+   ```bash
+   cp e2e/screenshots/actual/*.png e2e/screenshots/baseline/
+   ```
+4. Or use the update command:
+   ```bash
+   UPDATE_BASELINE=true npm run test:e2e
+   ```
+
+## Naming convention:
+
+- `{page-name}-desktop.png` - Desktop viewport
+- `{page-name}-mobile.png` - Mobile viewport
+- `{page-name}-tablet.png` - Tablet viewport
+- `{page-name}-{browser}.png` - Browser-specific
+- `{page-name}-dark.png` - Dark mode variant
+
+## Important:
+
+- Only commit stable, approved screenshots
+- Update baselines when UI intentionally changes
+- Review diffs carefully before updating