docs: create developer setup and contribution guide

docs/DEVELOPER.md

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+# Developer Setup & Contribution Guide
+
+This guide provides step-by-step instructions to set up the CodeLens development environment on your local machine.
+
+---
+
+## 🛠️ Prerequisites
+
+Make sure you have the following tools installed:
+- **Node.js** (v18.x or higher)
+- **npm** (v9.x or higher)
+- **MongoDB** (Local instance or MongoDB Atlas cluster)
+- **Git**
+
+---
+
+## 📂 Project Structure
+
+CodeLens is structured as a monorepo consisting of:
+- [frontend/](file:///c:/Users/babin/Desktop/GSSoC_2026/CodeLens/frontend) - Vite-powered React client.
+- [server/](file:///c:/Users/babin/Desktop/GSSoC_2026/CodeLens/server) - Node.js / Express backend API server.
+
+---
+
+## 🚀 Setting Up the Backend
+
+1. Navigate to the `server` directory:
+   ```bash
+   cd server
+   ```
+
+2. Install backend dependencies:
+   ```bash
+   npm install
+   ```
+
+3. Configure environment variables. Copy `.env.example` to `.env`:
+   ```bash
+   cp .env.example .env
+   ```
+
+4. Open `.env` and fill in the required parameters:
+   - `PORT`: Server port (e.g., `5000`)
+   - `MONGO_URI`: MongoDB connection string (e.g., `mongodb://localhost:27017/codelens` or MongoDB Atlas URI)
+   - `JWT_SECRET`: Secret key for JWT generation (at least 8 characters)
+   - `GEMINI_API_KEY`: API key for Gemini AI features
+   - `CLIENT_URL`: URL of the frontend (e.g., `http://localhost:5173`)
+   - `SMTP_HOST`, `SMTP_PORT`, `SMTP_USER`, `SMTP_PASS`: Mail credentials for OTP/notifications.
+
+5. Start the backend server in development mode:
+   ```bash
+   npm run dev
+   ```
+
+---
+
+## 💻 Setting Up the Frontend
+
+1. Navigate to the `frontend` directory:
+   ```bash
+   cd ../frontend
+   ```
+
+2. Install frontend dependencies:
+   ```bash
+   npm install
+   ```
+
+3. Configure local development environmental keys if needed. The frontend connects to `http://localhost:5000` by default.
+
+4. Start the frontend development server:
+   ```bash
+   npm run dev
+   ```
+
+5. Open your browser and navigate to `http://localhost:5173`.
+
+---
+
+## 🧪 Linting and Formatting
+
+Before submitting a Pull Request, ensure that your code complies with the project's standards by running linting checks:
+
+- **Frontend Linting**:
+  ```bash
+  cd frontend
+  npm run lint
+  ```
+- **Frontend Building**:
+  ```bash
+  npm run build
+  ```