docs: add code guidelines for backend and frontend development

Signed-off-by: Deluan <deluan@navidrome.org>

.github/copilot-instructions.md

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+# Navidrome Code Guidelines
+
+This is a music streaming server written in Go with a React frontend. The application manages music libraries, provides streaming capabilities, and offers various features like artist information, artwork handling, and external service integrations.
+
+## Code Standards
+
+### Backend (Go)
+- Follow standard Go conventions and idioms
+- Use context propagation for cancellation signals
+- Write unit tests for new functionality using Ginkgo/Gomega
+- Use mutex appropriately for concurrent operations
+- Implement interfaces for dependencies to facilitate testing
+
+### Frontend (React)
+- Use functional components with hooks
+- Follow React best practices for state management
+- Implement PropTypes for component properties
+- Prefer using React-Admin and Material-UI components
+- Icons should be imported from `react-icons` only
+- Follow existing patterns for API interaction
+
+## Repository Structure
+- `core/`: Server-side business logic (artwork handling, playback, etc.)
+- `ui/`: React frontend components
+- `model/`: Data models and repository interfaces
+- `server/`: API endpoints and server implementation
+- `utils/`: Shared utility functions
+- `persistence/`: Database access layer
+- `scanner/`: Music library scanning functionality
+
+## Key Guidelines
+1. Maintain cache management patterns for performance
+2. Follow the existing concurrency patterns (mutex, atomic)
+3. Use the testing framework appropriately (Ginkgo/Gomega for Go)
+4. Keep UI components focused and reusable
+5. Document configuration options in code
+6. Consider performance implications when working with music libraries
+7. Follow existing error handling patterns
+8. Ensure compatibility with external services (LastFM, Spotify)
+
+## Development Workflow
+- Test changes thoroughly, especially around concurrent operations
+- Validate both backend and frontend interactions
+- Consider how changes will affect user experience and performance
+- Test with different music library sizes and configurations