Add CONTRIBUTING guide for frontend setup and onboarding

CONTRIBUTING.md.

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+# Contributing to GreenCode
+
+Thank you for your interest in contributing to GreenCode! This guide will help you get started with both the backend and the React frontend.
+
+---
+
+##  Prerequisites
+
+Before you begin, ensure you have the following installed:
+
+### Backend (Spring Boot)
+- Java 17+
+- Maven 3.8+
+- PostgreSQL 14+
+- Docker & Docker Compose (recommended)
+
+### Frontend (React)
+- Node.js 18+
+- npm 9+ or yarn 1.22+
+
+---
+
+##  Setting Up the Frontend
+
+The GreenCode frontend is a React application located in the `greencode-frontend/` directory.
+
+### 1. Clone your fork
+
+```bash
+git clone https://github.com/<your-username>/GreenCode.git
+cd GreenCode/greencode-frontend
+```
+
+### 2. Install dependencies
+
+```bash
+npm install
+```
+
+### 3. Configure environment variables
+
+Copy the example environment file and update it:
+
+```bash
+cp .env.example .env
+```
+
+Update `.env` with your backend URL:
+### 4. Start the development server
+
+```bash
+npm start
+```
+
+The app will run at `http://localhost:3000`.
+
+---
+
+##  Connecting to the Backend
+
+Make sure the Spring Boot backend is running before starting the frontend. You can start it with:
+
+```bash
+# From the project root
+docker-compose up -d
+```
+
+Or manually:
+
+```bash
+mvn spring-boot:run
+```
+
+The backend runs at `http://localhost:8080` by default.
+
+---
+
+##  Frontend Structure
+---
+
+##  Git Workflow
+
+1. Fork the repository
+2. Create a feature branch: `git checkout -b feature/your-feature-name`
+3. Make your changes and commit: `git commit -m "feat: describe your change"`
+4. Push to your fork: `git push origin feature/your-feature-name`
+5. Open a Pull Request to `bos-com/GreenCode` main branch
+
+---
+
+##  Code Standards
+
+- Follow React functional component patterns with hooks
+- Use Axios for all API calls (configured in `src/services/`)
+- Keep components small and reusable
+- Write meaningful commit messages using [Conventional Commits](https://www.conventionalcommits.org/)
+
+---
+
+##  Reporting Issues
+
+If you find a bug or have a feature request, please open an issue at:
+https://github.com/bos-com/GreenCode/issues
+
+---
+
+##  Questions?
+
+Reach out via GitHub Discussions or open an issue tagged `question`.