Add a manipulator demo using Electron + rclnodejs (#1264)

This PR adds a comprehensive manipulator demo to the Electron examples, showcasing a two-joint robotic arm visualization using rclnodejs and Three.js. The demo provides interactive control through sliders, automatic animation modes, and real-time ROS2 integration.

Key changes:
- Adds a complete Two-Joint Manipulator demo application with 3D visualization
- Updates README documentation to feature both turtle_tf2 and manipulator demos in a structured table format
- Implements ROS2 joint state publishing/subscribing with visual feedback

Fix: #1263

Author: minggangw

README.md

@@ -78,9 +78,12 @@ API documentation is available [online](https://robotwebtools.github.io/rclnodej
 
 Create rich, interactive desktop applications using Electron and web technologies like Three.js. Build 3D visualizations, monitoring dashboards, and control interfaces that run on Windows, macOS, and Linux.
 
-Try the `electron_demo/turtle_tf2` demo for real-time coordinate frame visualization with dynamic transforms and keyboard-controlled turtle movement. More examples in [electron_demo](https://github.com/RobotWebTools/rclnodejs/tree/develop/electron_demo).
+|                       Demo                        |                                                              Description                                                              |                            Screenshot                            |
+| :-----------------------------------------------: | :-----------------------------------------------------------------------------------------------------------------------------------: | :--------------------------------------------------------------: |
+|  **🐢 [turtle_tf2](./electron_demo/turtle_tf2)**  |  Real-time coordinate frame visualization with turtle control. Features TF2 transforms, keyboard control, and dynamic frame updates.  |  ![turtle_tf2](./electron_demo/turtle_tf2/turtle-tf2-demo.png)   |
+| **🦾 [manipulator](./electron_demo/manipulator)** | Interactive two-joint robotic arm simulation. Features 3D joint visualization, manual/automatic control, and visual movement markers. | ![manipulator](./electron_demo/manipulator/manipulator-demo.png) |
 
-![demo screenshot](./electron_demo/turtle_tf2/turtle-tf2-demo.png)
+Explore more examples in [electron_demo](https://github.com/RobotWebTools/rclnodejs/tree/develop/electron_demo).
 
 ## Using rclnodejs with TypeScript
 

electron_demo/manipulator/README.md

@@ -0,0 +1,300 @@
+# Two-Joint Manipulator Demo
+
+An interactive Electron application demonstrating a two-joint robotic manipulator visualization using rclnodejs and Three.js.
+
+![Manipulator Demo](./manipulator-demo.gif)
+
+## 🚀 Features
+
+- **Real-time 3D Visualization**: Interactive two-joint robotic arm with Three.js
+- **ROS2 Integration**: Publishes and subscribes to `sensor_msgs/msg/JointState` topics
+- **Interactive Control**: Manual joint control with sliders
+- **Automatic Animation**: Smooth sinusoidal motion patterns
+- **Live Feedback**: Real-time joint position display and ROS2 message frequency
+- **Visual Movement Markers**: Color-coded rings, arrows, and labels to identify joint movements
+- **Modern UI**: Clean, responsive interface with 3D orbit controls
+
+## 📋 Prerequisites
+
+- **Node.js** (>= 16.13.0) - JavaScript runtime
+- **ROS 2** (Humble, Jazzy, or newer) - Robot Operating System 2
+- **rclnodejs compatible environment** - Linux recommended (tested on Ubuntu/WSL)
+
+## 🛠️ Installation
+
+1. **Navigate to the demo directory**:
+
+   ```bash
+   cd rclnodejs/electron_demo/manipulator
+   ```
+
+2. **Install dependencies**:
+
+   ```bash
+   npm install
+   ```
+
+3. **Rebuild native modules for Electron**:
+   ```bash
+   npm run rebuild
+   ```
+
+## 📜 Available Scripts
+
+- **`npm start`** - Run demo (requires manual ROS2 environment setup)
+- **`npm run rebuild`** - Rebuild native modules after dependency changes
+
+## 🚀 Quick Start
+
+### Option 1: Simple Demo (Works Everywhere)
+
+```bash
+npm start
+```
+
+- ✅ **No ROS2 environment required**
+- ✅ **Works without external setup**
+- ✅ **Pure visualization and manual control**
+- ⚠️ No ROS2 topic publishing (local mode only)
+
+### Option 2: Manual ROS2 Setup (Recommended for ROS2 Integration)
+
+1. **Source your ROS2 environment**:
+
+   ```bash
+   source /opt/ros/humble/setup.bash  # or your ROS2 installation path
+   ```
+
+2. **Run the demo**:
+   ```bash
+   npm start
+   ```
+
+- ✅ **Publishes to `/joint_states` topic**
+- ✅ **Full ROS2 ecosystem integration**
+- ✅ **Real-time ROS2 message monitoring**
+
+## 🎮 Usage
+
+### Interactive Controls
+
+- **Joint Sliders**: Use the sliders in the control panel to manually adjust joint angles
+
+  - **Joint 1 (Base)**: Rotates the entire arm around the vertical axis (±180°)
+  - **Joint 2 (Elbow)**: Bends the upper arm segment (±135°)
+
+- **Animation**: Click "Start Animation" for automatic smooth motion
+- **Reset**: Click "Reset Position" to return to zero configuration
+
+### 3D Visualization Controls
+
+- **Orbit**: Click and drag to rotate the camera around the manipulator
+- **Zoom**: Use mouse wheel to zoom in/out
+- **View**: The manipulator is shown with color-coded components:
+  - **Gray Base**: Fixed mounting base
+  - **Red Joint 1**: Base rotation joint
+  - **Blue Link 1**: Upper arm segment
+  - **Green Joint 2**: Elbow rotation joint
+  - **Yellow Link 2**: Forearm segment
+  - **Purple End Effector**: Tool attachment point
+
+### Visual Movement Markers
+
+The demo includes visual indicators to help identify joint movements:
+
+- **🔴 Red Markers**: Joint 1 (Base rotation)
+
+  - Red ring around the base joint
+  - Red arrow showing rotation direction
+  - "Joint1" text label
+
+- **🟢 Green Markers**: Joint 2 (Elbow)
+
+  - Green ring around the elbow joint
+  - Green arrow showing rotation direction
+  - "Joint2" text label
+
+- **⚪ White Reference Ring**: Fixed base reference point
+
+These markers make it easy to visually confirm which joints are moving during manual control or animation.
+
+## 🔧 ROS2 Topics
+
+### Published Topics
+
+- **`/joint_states`** (`sensor_msgs/msg/JointState`)
+  - Joint names: `['joint1', 'joint2']`
+  - Positions: Current joint angles in radians
+  - Published at 10 Hz
+  - Velocity and effort fields included (set to zero)
+
+### Subscribed Topics
+
+- **`/joint_states`** (`sensor_msgs/msg/JointState`)
+  - Receives external joint commands
+  - Updates visualization in real-time
+  - Displays message frequency and count
+
+### Monitoring Topics
+
+To verify the demo is working correctly, you can monitor the published topics:
+
+```bash
+# In a separate terminal, source ROS2 environment
+source /opt/ros/humble/setup.bash  # or your ROS2 installation
+
+# List all available topics
+ros2 topic list
+
+# Monitor joint state messages
+ros2 topic echo /joint_states
+
+# Check publishing frequency
+ros2 topic hz /joint_states
+
+# View topic info
+ros2 topic info /joint_states
+```
+
+## 🏗️ Architecture
+
+### Main Process (`main.js`)
+
+- **ROS2 Node Management**: Creates and manages the manipulator node
+- **Joint State Publishing**: Publishes current joint positions at 10 Hz
+- **Animation Control**: Handles smooth motion generation
+- **IPC Communication**: Bridges ROS2 data with renderer process
+
+### Renderer Process (`renderer.js`)
+
+- **3D Visualization**: Three.js scene with interactive manipulator model
+- **Visual Markers**: Color-coded rings, arrows, and labels for joint identification
+- **User Interface**: Control panels and status displays
+- **Real-time Updates**: Smooth joint motion and camera controls
+- **Message Handling**: Processes ROS2 data and user interactions
+
+### Component Hierarchy
+
+```
+Scene
+├── Lighting (Ambient + Directional + Point)
+├── Ground Plane
+├── Coordinate Axes
+├── Base (Fixed)
+├── Joint Markers (Red/Green Rings, Arrows, Labels)
+└── Joint1 Group (Rotates around Y-axis)
+    ├── Joint1 Sphere
+    ├── Link1 Cylinder
+    └── Joint2 Group (Rotates around Z-axis)
+        ├── Joint2 Sphere
+        ├── Link2 Cylinder
+        └── End Effector Cube
+```
+
+## 🎯 Technical Details
+
+### Joint Configuration
+
+- **Joint 1 (Base)**: Revolute joint, Y-axis rotation, ±180° range
+- **Joint 2 (Elbow)**: Revolute joint, Z-axis rotation, ±135° range
+- **Forward Kinematics**: Hierarchical transformation chains
+- **Coordinate System**: Right-handed, Z-up convention
+
+### Animation Patterns
+
+```javascript
+// Sinusoidal motion with different frequencies (from main.js)
+joint1_angle = (sin(time * 1.0) * π) / 3; // ±60° at 1.0x speed
+joint2_angle = (sin(time * 1.5) * π) / 4; // ±45° at 1.5x speed
+```
+
+### Performance
+
+- **Rendering**: 60 FPS with requestAnimationFrame
+- **ROS2 Publishing**: 10 Hz joint state updates
+- **Animation**: 20 Hz smooth motion updates
+- **UI Updates**: Real-time slider and display synchronization
+
+## 🛠️ Development
+
+### File Structure
+
+```
+manipulator/
+├── package.json          # Dependencies and scripts
+├── main.js              # Electron main process + ROS2 integration
+├── renderer.js          # Three.js visualization + UI controls
+├── index.html           # Application UI layout and styling
+├── start-demo.sh        # Convenient ROS2 startup script
+├── README.md           # This documentation
+├── VERSION_UPGRADE.md  # rclnodejs upgrade details
+└── COMPLETION_SUMMARY.md # Implementation summary
+```
+
+### Key Dependencies
+
+- **electron**: `^31.7.7` - Desktop application framework
+- **rclnodejs**: `^1.5.1` - ROS2 JavaScript client library (latest compatible)
+- **@electron/rebuild**: `^3.7.2` - Native module rebuilding tool
+- **three.js**: `r128` - 3D graphics library (loaded via CDN)
+
+### Debugging
+
+- Press `F12` to open Developer Tools
+- Console logs show ROS2 initialization and message flow
+- Check ROS2 topics with: `ros2 topic list` and `ros2 topic echo /joint_states`
+
+## 🔍 Troubleshooting
+
+### Common Issues
+
+1. **"librcl.so not found"**
+
+   ```bash
+   # Source ROS2 environment manually
+   source /opt/ros/humble/setup.bash  # or your ROS2 installation path
+   npm start
+   ```
+
+2. **Build errors with latest Electron**
+
+   - This demo uses Electron 31.7.7 for rclnodejs compatibility
+   - Electron 38+ requires C++20, which rclnodejs doesn't support yet
+   - The current versions are tested and stable
+
+3. **No ROS2 messages received**
+
+   - Check if ROS2 daemon is running: `ros2 daemon start`
+   - Verify topic exists: `ros2 topic list`
+   - Check message flow: `ros2 topic echo /joint_states`
+
+4. **Blank 3D scene**
+   - Check browser console for Three.js errors
+   - Ensure internet connection for Three.js CDN
+   - Press F12 to open Developer Tools for debugging
+
+### Performance Tips
+
+- Reduce animation frequency for slower systems
+- Disable shadows in Three.js for better performance
+- Adjust rendering quality in renderer settings
+
+## 📚 Learning Resources
+
+- **ROS2 Concepts**: [ROS2 Documentation](https://docs.ros.org/en/humble/)
+- **Three.js Guide**: [Three.js Documentation](https://threejs.org/docs/)
+- **Electron Tutorials**: [Electron Documentation](https://electronjs.org/docs)
+- **rclnodejs API**: [rclnodejs Repository](https://github.com/RobotWebTools/rclnodejs)
+
+## 🤝 Contributing
+
+This demo is part of the rclnodejs project. Contributions welcome!
+
+1. Fork the repository
+2. Create your feature branch
+3. Add tests for new functionality
+4. Submit a pull request
+
+## 📄 License
+
+Licensed under the Apache License, Version 2.0. See the main rclnodejs repository for details.