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+# Source Academy Frontend
+
+The Source Academy Frontend is a React.js TypeScript web application for an immersive online programming education platform. It features a coding playground, SICP JS textbook integration, assessment system, grading tools, collaborative sessions, and game elements. The application is built with React 18, Redux, TypeScript, and modern tooling.
+
+**ALWAYS reference these instructions first and fallback to search or bash commands only when you encounter unexpected information that does not match the info here.**
+
+## Working Effectively
+
+### Environment Setup
+
+- Node.js version: 20.9.0+ (as specified in `.node-version`)
+- Package manager: Yarn 4.6.0 (managed via corepack)
+- Enable corepack: `corepack enable`
+
+### Bootstrap and Build Process
+
+1. **Install dependencies** (25 seconds):
+
+   ```bash
+   corepack enable
+   yarn install
+   ```
+
+   **NOTE**: Installation may fail due to network restrictions accessing `cdn.sheetjs.com` for the xlsx package. If this occurs, temporarily modify `package.json` to use `"xlsx": "^0.18.5"` instead of the CDN URL, run `yarn install --mode update-lockfile`, then `yarn install`.
+
+2. **TypeScript compilation** (20 seconds):
+
+   ```bash
+   yarn run tsc --noEmit
+   ```
+
+3. **Linting** (7 seconds):
+
+   ```bash
+   yarn run eslint
+   ```
+
+4. **Format checks** (10 seconds):
+
+   ```bash
+   yarn run format:ci
+   ```
+
+5. **Production build** (40 seconds, **NEVER CANCEL**):
+
+   ```bash
+   yarn run build
+   ```
+
+   **CRITICAL**: Set timeout to 60+ minutes. Build completes in ~40 seconds but may take longer on slower systems.
+
+6. **Test suite** (1 minute 45 seconds, **NEVER CANCEL**):
+
+   ```bash
+   yarn run test
+   ```
+
+   **CRITICAL**: Set timeout to 30+ minutes. One test may fail due to network restrictions accessing `sicp.sourceacademy.org`.
+
+### Development Server
+
+- **Start development server** (16 seconds build + server startup):
+
+  ```bash
+  yarn run start
+  ```
+
+- **Access**: <http://localhost:8000>
+- **Hot reload**: Enabled with live updates
+
+## Validation
+
+### Manual Testing Requirements
+
+**ALWAYS run through at least one complete end-to-end scenario after making changes:**
+
+1. **Basic Playground Functionality**:
+   - Navigate to <http://localhost:8000> (redirects to /playground)
+   - Verify the editor loads with syntax highlighting
+   - Write test code: `function factorial(n) { return n <= 1 ? 1 : n * factorial(n - 1); } factorial(5);`
+   - Click "Run" button
+   - Verify output "120" appears in the result panel
+   - Test REPL by typing expressions in the bottom panel
+
+2. **Navigation and UI**:
+   - Verify navigation bar with "Playground" and "SICP JS" links
+   - Test language selector (Source §1 through §4)
+   - Verify all toolbar buttons are functional (Run, Share, Session, Folder, Google Drive, GitHub)
+   - Check welcome message and documentation links work
+
+3. **Build Validation**:
+   - **ALWAYS** run `yarn run format` and `yarn run eslint` before committing
+   - The CI workflow (`.github/workflows/ci.yml`) runs: `tsc`, `format:ci`, `eslint`, `build`, `test --bail=1`, `test-coveralls`
+
+## Network Limitations
+
+**Important**: Several external services are not accessible in sandboxed environments:
+
+- `cdn.sheetjs.com` - xlsx package installation fails
+- `sicp.sourceacademy.org` - SICP JS content fails to load
+- `fonts.googleapis.com`, `apis.google.com` - Google services blocked
+- These failures are expected and should be documented as "fails due to network limitations"
+
+## Common Tasks
+
+### Repository Structure
+
+```
+/home/runner/work/frontend/frontend/
+├── .github/workflows/        # CI/CD pipelines
+├── .node-version            # Node.js 20.9.0
+├── package.json             # Dependencies and scripts
+├── tsconfig.json           # TypeScript configuration
+├── rsbuild.config.ts       # Build configuration (replaces webpack)
+├── vitest.config.ts        # Test configuration
+├── src/
+│   ├── assets/             # Static assets
+│   ├── commons/            # Shared components and utilities
+│   ├── features/           # Redux features (actions, reducers, sagas)
+│   ├── pages/              # Page components matching routes
+│   └── styles/             # SCSS stylesheets
+├── public/                 # Public static files
+└── scripts/                # Utility scripts
+```
+
+### Key npm Scripts
+
+- `yarn start` - Development server (<http://localhost:8000>)
+- `yarn build` - Production build (40s, outputs to `build/`)
+- `yarn test` - Test suite (1m45s, may have 1 network-related failure)
+- `yarn format` - Auto-fix code formatting (ESLint + Prettier)
+- `yarn format:ci` - Check formatting without changes
+- `yarn eslint` - Lint TypeScript/JavaScript code
+- `yarn tsc` - TypeScript compilation check
+
+### Environment Configuration
+
+Copy `.env.example` to `.env` and configure:
+
+- `REACT_APP_BACKEND_URL` - Backend API endpoint
+- `REACT_APP_USE_BACKEND` - Enable/disable backend integration
+- `REACT_APP_PLAYGROUND_ONLY` - Playground-only mode for GitHub Pages
+- Authentication providers, Google Drive, GitHub integration settings
+
+### Memory Requirements
+
+The build process requires significant memory:
+
+- Set `NODE_OPTIONS=--max_old_space_size=8192` for CI/CD
+- Local development typically works with default Node.js memory limits
+
+### Technology Stack
+
+- **Frontend**: React 18.3.1 with TypeScript 5.8.2
+- **State Management**: Redux Toolkit with Redux Saga
+- **Build Tool**: rsbuild (modern webpack replacement)
+- **Testing**: Vitest with jsdom environment
+- **Styling**: SCSS with Blueprint UI components
+- **Linting**: ESLint 9 with TypeScript rules
+- **Code Editor**: ACE editor with Source language support
+
+### Debugging Tips
+
+- Check browser console for React DevTools and HMR connection messages
+- Build warnings about missing Node.js modules (perf_hooks) are expected in browser builds
+- ESLint warnings about React hooks dependencies are common and generally safe
+- Use `yarn run test:watch` for interactive test development
+- Use `yarn run test:ui` for browser-based test UI
+
+**Remember**: Always validate functionality manually by running the application and testing user scenarios. The playground must be able to execute code and display results correctly.
diff --git a/.markdownlint.jsonc b/.markdownlint.jsonc
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+{
+    "line-length": false
+}