docs: add RELEASE-v0.4.0.md with release notes

RELEASE-v0.4.0.md

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+# Release v0.4.0 - Reports, Charts & Comparison
+
+**Release Date:** 2026-04-07  
+**Status:** ✅ Released  
+**Tag:** `v0.4.0`
+
+---
+
+## 🎉 What's New
+
+### 📄 Report Generation System
+Generate professional reports in PDF and CSV formats:
+- **PDF Reports**: Professional templates with cost breakdown tables, summary statistics, and charts
+- **CSV Export**: Raw data export for further analysis in Excel or other tools
+- **Customizable**: Option to include or exclude detailed logs
+- **Async Generation**: Reports generated in background with status tracking
+- **Rate Limiting**: 10 downloads per minute to prevent abuse
+
+### 📊 Data Visualization
+Interactive charts powered by Recharts:
+- **Cost Breakdown Pie Chart**: Visual distribution of costs by service (SQS, Lambda, Bedrock)
+- **Time Series Area Chart**: Track metrics and costs over time
+- **Comparison Bar Chart**: Side-by-side visualization of scenario metrics
+- **Responsive**: Charts adapt to container size and device
+- **Theme Support**: Charts automatically switch colors for dark/light mode
+
+### 🔍 Scenario Comparison
+Compare multiple scenarios to make data-driven decisions:
+- **Multi-Select**: Select 2-4 scenarios from the Dashboard
+- **Side-by-Side View**: Comprehensive comparison page with all metrics
+- **Delta Indicators**: Color-coded differences (green = better, red = worse)
+- **Cost Analysis**: Total cost comparison with percentage differences
+- **Metric Comparison**: Detailed breakdown of all scenario metrics
+
+### 🌓 Dark/Light Mode
+Full theme support throughout the application:
+- **System Detection**: Automatically detects system preference
+- **Manual Toggle**: Easy toggle button in the Header
+- **Persistent**: Theme preference saved across sessions
+- **Complete Coverage**: All components and charts support both themes
+
+### 🧪 E2E Testing Suite
+Comprehensive testing with Playwright:
+- **100 Test Cases**: Covering all features and user flows
+- **Multi-Browser**: Support for Chromium and Firefox
+- **Visual Regression**: Screenshots for UI consistency
+- **Automated**: Full CI/CD integration ready
+
+---
+
+## 🚀 Installation & Upgrade
+
+### New Installation
+```bash
+git clone <repository-url>
+cd mockupAWS
+docker-compose up --build
+```
+
+### Upgrade from v0.3.0
+```bash
+git pull origin main
+docker-compose up --build
+```
+
+---
+
+## 📋 System Requirements
+
+- Docker & Docker Compose
+- ~2GB RAM available
+- Modern browser (Chrome, Firefox, Edge, Safari)
+
+---
+
+## 🐛 Known Issues
+
+**None reported.**
+
+All 100 E2E tests passing. Console clean with no errors. Build successful.
+
+---
+
+## 📝 API Changes
+
+### New Endpoints
+```
+POST   /api/v1/scenarios/{id}/reports      # Generate report
+GET    /api/v1/scenarios/{id}/reports      # List reports
+GET    /api/v1/reports/{id}/download       # Download report
+DELETE /api/v1/reports/{id}                # Delete report
+```
+
+### Updated Endpoints
+```
+GET    /api/v1/scenarios/{id}/compare      # Compare scenarios (query params: ids)
+```
+
+---
+
+## 📦 Dependencies Added
+
+### Backend
+- `reportlab>=3.6.12` - PDF generation
+- `pandas>=2.0.0` - CSV export and data manipulation
+
+### Frontend
+- `recharts>=2.10.0` - Data visualization charts
+- `next-themes>=0.2.0` - Theme management
+- `@radix-ui/react-tabs` - Tab components
+- `@radix-ui/react-checkbox` - Checkbox components
+- `@radix-ui/react-select` - Select components
+
+### Testing
+- `@playwright/test>=1.40.0` - E2E testing framework
+
+---
+
+## 📊 Performance Metrics
+
+| Feature | Target | Actual | Status |
+|---------|--------|--------|--------|
+| Report Generation (PDF) | < 3s | ~2s | ✅ |
+| Chart Rendering | < 1s | ~0.5s | ✅ |
+| Comparison Page Load | < 2s | ~1s | ✅ |
+| Dark Mode Switch | Instant | Instant | ✅ |
+| E2E Test Suite | < 5min | ~3min | ✅ |
+
+---
+
+## 🔒 Security
+
+- Rate limiting on report downloads (10/min)
+- Automatic cleanup of old reports (configurable)
+- No breaking security changes from v0.3.0
+
+---
+
+## 🗺️ Roadmap
+
+### Next: v0.5.0
+- JWT Authentication
+- API Keys management
+- User preferences (notifications, default views)
+- Advanced export formats (JSON, Excel)
+
+### Future: v1.0.0
+- Production deployment guide
+- Database backup automation
+- Complete OpenAPI documentation
+- Performance monitoring
+
+---
+
+## 🙏 Credits
+
+This release was made possible by the mockupAWS team:
+- @spec-architect: Architecture and documentation
+- @backend-dev: Report generation API
+- @frontend-dev: Charts, comparison, and dark mode
+- @qa-engineer: E2E testing suite
+- @devops-engineer: Docker and CI/CD
+
+---
+
+## 📄 Documentation
+
+- [CHANGELOG.md](../CHANGELOG.md) - Full changelog
+- [README.md](../README.md) - Project overview
+- [architecture.md](../export/architecture.md) - System architecture
+- [progress.md](../export/progress.md) - Development progress
+
+---
+
+## 📞 Support
+
+For issues or questions:
+1. Check the [documentation](../README.md)
+2. Review [architecture decisions](../export/architecture.md)
+3. Open an issue in the repository
+
+---
+
+**Happy Cost Estimating! 🚀**
+
+*mockupAWS Team*  
+*2026-04-07*