docs: add comprehensive camera switching implementation guide

Add detailed documentation for camera switching functionality across web and mobile platforms:

- Add camera management interfaces to QRScannerService
- Document MLKit Barcode Scanner configuration for Capacitor
- Add platform-specific implementations for iOS and Android
- Include camera state management and error handling
- Add performance optimization guidelines
- Document testing requirements and scenarios

Key additions:
- Camera switching implementation for both platforms
- Platform-specific considerations (iOS/Android)
- Battery and memory optimization strategies
- Comprehensive testing guidelines
- Error handling and state management
- Security and permission considerations

This update provides a complete reference for implementing robust
camera switching functionality in the QR code scanner.

doc/qr-code-implementation-guide.md

@@ -54,6 +54,9 @@ interface QRScannerService {
   addListener(listener: ScanListener): void;
   onStream(callback: (stream: MediaStream | null) => void): void;
   cleanup(): Promise<void>;
+  getAvailableCameras(): Promise<MediaDeviceInfo[]>;
+  switchCamera(deviceId: string): Promise<void>;
+  getCurrentCamera(): Promise<MediaDeviceInfo | null>;
 }
 
 interface ScanListener {
@@ -87,7 +90,15 @@ const config: CapacitorConfig = {
       formats: ['QR_CODE'],
       detectorSize: 1.0,
       lensFacing: 'back',
-      googleBarcodeScannerModuleInstallState: true
+      googleBarcodeScannerModuleInstallState: true,
+      // Additional camera options
+      cameraOptions: {
+        quality: 0.8,
+        allowEditing: false,
+        resultType: 'uri',
+        sourceType: 'CAMERA',
+        saveToGallery: false
+      }
     }
   }
 };
@@ -281,4 +292,514 @@ export async function createBuildConfig(mode: string) {
 - Track memory usage
 - Monitor camera performance
 - Check error rates
-- Analyze user feedback
+- Analyze user feedback
+
+## Camera Handling
+
+### Camera Switching Implementation
+
+The QR scanner supports camera switching on both mobile and desktop platforms through a unified interface.
+
+#### Platform-Specific Implementations
+
+1. **Mobile (Capacitor)**
+   - Uses `@capacitor-mlkit/barcode-scanning`
+   - Supports front/back camera switching
+   - Native camera access through platform APIs
+   - Optimized for mobile performance
+
+   ```typescript
+   // CapacitorQRScanner.ts
+   async startScan(options?: QRScannerOptions): Promise<void> {
+     const scanOptions: StartScanOptions = {
+       formats: [BarcodeFormat.QrCode],
+       lensFacing: options?.camera === "front" ? 
+         LensFacing.Front : LensFacing.Back
+     };
+     await BarcodeScanner.startScan(scanOptions);
+   }
+   ```
+
+2. **Web (Desktop)**
+   - Uses browser's MediaDevices API
+   - Supports multiple camera devices
+   - Dynamic camera enumeration
+   - Real-time camera switching
+
+   ```typescript
+   // WebInlineQRScanner.ts
+   async getAvailableCameras(): Promise<MediaDeviceInfo[]> {
+     const devices = await navigator.mediaDevices.enumerateDevices();
+     return devices.filter(device => device.kind === 'videoinput');
+   }
+
+   async switchCamera(deviceId: string): Promise<void> {
+     // Stop current stream
+     await this.stopScan();
+     
+     // Start new stream with selected camera
+     this.stream = await navigator.mediaDevices.getUserMedia({
+       video: {
+         deviceId: { exact: deviceId },
+         width: { ideal: 1280 },
+         height: { ideal: 720 }
+       }
+     });
+     
+     // Update video and restart scanning
+     if (this.video) {
+       this.video.srcObject = this.stream;
+       await this.video.play();
+     }
+     this.scanQRCode();
+   }
+   ```
+
+### Core Interfaces
+
+```typescript
+interface QRScannerService {
+  // ... existing methods ...
+
+  /** Get available cameras */
+  getAvailableCameras(): Promise<MediaDeviceInfo[]>;
+  
+  /** Switch to a specific camera */
+  switchCamera(deviceId: string): Promise<void>;
+  
+  /** Get current camera info */
+  getCurrentCamera(): Promise<MediaDeviceInfo | null>;
+}
+
+interface QRScannerOptions {
+  /** Camera to use ('front' or 'back' for mobile) */
+  camera?: "front" | "back";
+  /** Whether to show a preview of the camera feed */
+  showPreview?: boolean;
+  /** Whether to play a sound on successful scan */
+  playSound?: boolean;
+}
+```
+
+### UI Components
+
+The camera switching UI adapts to the platform:
+
+1. **Mobile Interface**
+   - Simple toggle button for front/back cameras
+   - Positioned in bottom-right corner
+   - Clear visual feedback during switching
+   - Native camera controls
+
+   ```vue
+   <button 
+     v-if="isNativePlatform"
+     @click="toggleMobileCamera"
+     class="camera-switch-btn"
+   >
+     <font-awesome icon="camera-rotate" />
+     Switch Camera
+   </button>
+   ```
+
+2. **Desktop Interface**
+   - Dropdown menu with all available cameras
+   - Camera labels and device IDs
+   - Real-time camera switching
+   - Responsive design
+
+   ```vue
+   <select 
+     v-model="selectedCameraId"
+     @change="onCameraChange"
+     class="camera-select-dropdown"
+   >
+     <option 
+       v-for="camera in availableCameras" 
+       :key="camera.deviceId"
+       :value="camera.deviceId"
+     >
+       {{ camera.label || `Camera ${camera.deviceId.slice(0, 4)}` }}
+     </option>
+   </select>
+   ```
+
+### Error Handling
+
+The camera switching implementation includes comprehensive error handling:
+
+1. **Common Error Scenarios**
+   - Camera in use by another application
+   - Permission denied during switch
+   - Device not available
+   - Stream initialization failure
+   - Camera switch timeout
+
+2. **Error Response**
+   ```typescript
+   private async handleCameraSwitch(deviceId: string): Promise<void> {
+     try {
+       this.updateCameraState("initializing", "Switching camera...");
+       await this.switchCamera(deviceId);
+       this.updateCameraState("active", "Camera switched successfully");
+     } catch (error) {
+       this.updateCameraState("error", "Failed to switch camera");
+       throw error;
+     }
+   }
+   ```
+
+3. **User Feedback**
+   - Visual indicators during switching
+   - Error notifications
+   - Camera state updates
+   - Permission request dialogs
+
+### State Management
+
+The camera system maintains several states:
+
+1. **Camera States**
+   ```typescript
+   type CameraState =
+     | "initializing"  // Camera is being initialized
+     | "ready"        // Camera is ready to use
+     | "active"       // Camera is actively streaming
+     | "in_use"       // Camera is in use by another application
+     | "permission_denied" // Camera permission was denied
+     | "not_found"    // No camera found on device
+     | "error"        // Generic error state
+     | "off";         // Camera is off
+   ```
+
+2. **State Transitions**
+   - Initialization → Ready
+   - Ready → Active
+   - Active → Switching
+   - Switching → Active/Error
+   - Any state → Off (on cleanup)
+
+### Best Practices
+
+1. **Camera Access**
+   - Always check permissions before switching
+   - Handle camera busy states
+   - Implement proper cleanup
+   - Monitor camera state changes
+
+2. **Performance**
+   - Optimize camera resolution
+   - Handle stream switching efficiently
+   - Manage memory usage
+   - Implement proper cleanup
+
+3. **User Experience**
+   - Clear visual feedback
+   - Smooth camera transitions
+   - Intuitive camera controls
+   - Responsive UI updates
+   - Accessible camera selection
+
+4. **Security**
+   - Secure camera access
+   - Permission management
+   - Device validation
+   - Stream security
+
+### Testing
+
+1. **Test Scenarios**
+   - Camera switching on both platforms
+   - Permission handling
+   - Error conditions
+   - Multiple camera devices
+   - Camera busy states
+   - Stream initialization
+   - UI responsiveness
+
+2. **Test Environment**
+   - Multiple mobile devices
+   - Various desktop browsers
+   - Different camera configurations
+   - Network conditions
+   - Permission states
+
+### Capacitor Implementation Details
+
+#### MLKit Barcode Scanner Configuration
+
+1. **Plugin Setup**
+   ```typescript
+   // capacitor.config.ts
+   const config: CapacitorConfig = {
+     plugins: {
+       MLKitBarcodeScanner: {
+         formats: ['QR_CODE'],
+         detectorSize: 1.0,
+         lensFacing: 'back',
+         googleBarcodeScannerModuleInstallState: true,
+         // Additional camera options
+         cameraOptions: {
+           quality: 0.8,
+           allowEditing: false,
+           resultType: 'uri',
+           sourceType: 'CAMERA',
+           saveToGallery: false
+         }
+       }
+     }
+   };
+   ```
+
+2. **Camera Management**
+   ```typescript
+   // CapacitorQRScanner.ts
+   export class CapacitorQRScanner implements QRScannerService {
+     private currentLensFacing: LensFacing = LensFacing.Back;
+     
+     async getAvailableCameras(): Promise<MediaDeviceInfo[]> {
+       // On mobile, we have two fixed cameras
+       return [
+         {
+           deviceId: 'back',
+           label: 'Back Camera',
+           kind: 'videoinput'
+         },
+         {
+           deviceId: 'front',
+           label: 'Front Camera',
+           kind: 'videoinput'
+         }
+       ] as MediaDeviceInfo[];
+     }
+
+     async switchCamera(deviceId: string): Promise<void> {
+       if (!this.isScanning) return;
+       
+       const newLensFacing = deviceId === 'front' ? 
+         LensFacing.Front : LensFacing.Back;
+       
+       // Stop current scan
+       await this.stopScan();
+       
+       // Update lens facing
+       this.currentLensFacing = newLensFacing;
+       
+       // Restart scan with new camera
+       await this.startScan({
+         camera: deviceId as 'front' | 'back'
+       });
+     }
+
+     async getCurrentCamera(): Promise<MediaDeviceInfo | null> {
+       return {
+         deviceId: this.currentLensFacing === LensFacing.Front ? 'front' : 'back',
+         label: this.currentLensFacing === LensFacing.Front ? 
+           'Front Camera' : 'Back Camera',
+         kind: 'videoinput'
+       } as MediaDeviceInfo;
+     }
+   }
+   ```
+
+3. **Camera State Management**
+   ```typescript
+   // CapacitorQRScanner.ts
+   private async handleCameraState(): Promise<void> {
+     try {
+       // Check if camera is available
+       const { camera } = await BarcodeScanner.checkPermissions();
+       
+       if (camera === 'denied') {
+         this.updateCameraState('permission_denied');
+         return;
+       }
+       
+       // Check if camera is in use
+       const isInUse = await this.isCameraInUse();
+       if (isInUse) {
+         this.updateCameraState('in_use');
+         return;
+       }
+       
+       this.updateCameraState('ready');
+     } catch (error) {
+       this.updateCameraState('error', error.message);
+     }
+   }
+
+   private async isCameraInUse(): Promise<boolean> {
+     try {
+       // Try to start a test scan
+       await BarcodeScanner.startScan({
+         formats: [BarcodeFormat.QrCode],
+         lensFacing: this.currentLensFacing
+       });
+       // If successful, stop it immediately
+       await BarcodeScanner.stopScan();
+       return false;
+     } catch (error) {
+       return error.message.includes('camera in use');
+     }
+   }
+   ```
+
+4. **Error Handling**
+   ```typescript
+   // CapacitorQRScanner.ts
+   private async handleCameraError(error: Error): Promise<void> {
+     switch (error.name) {
+       case 'CameraPermissionDenied':
+         this.updateCameraState('permission_denied');
+         break;
+       case 'CameraInUse':
+         this.updateCameraState('in_use');
+         break;
+       case 'CameraUnavailable':
+         this.updateCameraState('not_found');
+         break;
+       default:
+         this.updateCameraState('error', error.message);
+     }
+   }
+   ```
+
+#### Platform-Specific Considerations
+
+1. **iOS Implementation**
+   - Camera permissions in Info.plist
+   - Privacy descriptions
+   - Camera usage description
+   - Background camera access
+
+   ```xml
+   <!-- ios/App/App/Info.plist -->
+   <key>NSCameraUsageDescription</key>
+   <string>We need access to your camera to scan QR codes</string>
+   <key>NSPhotoLibraryUsageDescription</key>
+   <string>We need access to save scanned QR codes</string>
+   ```
+
+2. **Android Implementation**
+   - Camera permissions in AndroidManifest.xml
+   - Runtime permission handling
+   - Camera features declaration
+   - Hardware feature requirements
+
+   ```xml
+   <!-- android/app/src/main/AndroidManifest.xml -->
+   <uses-permission android:name="android.permission.CAMERA" />
+   <uses-feature android:name="android.hardware.camera" />
+   <uses-feature android:name="android.hardware.camera.autofocus" />
+   ```
+
+3. **Platform-Specific Features**
+   - iOS: Camera orientation handling
+   - Android: Camera resolution optimization
+   - Both: Battery usage optimization
+   - Both: Memory management
+
+   ```typescript
+   // Platform-specific optimizations
+   private getPlatformSpecificOptions(): StartScanOptions {
+     const baseOptions: StartScanOptions = {
+       formats: [BarcodeFormat.QrCode],
+       lensFacing: this.currentLensFacing
+     };
+
+     if (Capacitor.getPlatform() === 'ios') {
+       return {
+         ...baseOptions,
+         // iOS-specific options
+         cameraOptions: {
+           quality: 0.7, // Lower quality for better performance
+           allowEditing: false,
+           resultType: 'uri'
+         }
+       };
+     } else if (Capacitor.getPlatform() === 'android') {
+       return {
+         ...baseOptions,
+         // Android-specific options
+         cameraOptions: {
+           quality: 0.8,
+           allowEditing: false,
+           resultType: 'uri',
+           saveToGallery: false
+         }
+       };
+     }
+
+     return baseOptions;
+   }
+   ```
+
+#### Performance Optimization
+
+1. **Battery Usage**
+   ```typescript
+   // CapacitorQRScanner.ts
+   private optimizeBatteryUsage(): void {
+     // Reduce scan frequency when battery is low
+     if (this.isLowBattery()) {
+       this.scanInterval = 2000; // 2 seconds between scans
+     } else {
+       this.scanInterval = 1000; // 1 second between scans
+     }
+   }
+
+   private isLowBattery(): boolean {
+     // Check battery level if available
+     if (Capacitor.isPluginAvailable('Battery')) {
+       const { level } = await Battery.getBatteryLevel();
+       return level < 0.2; // 20% or lower
+     }
+     return false;
+   }
+   ```
+
+2. **Memory Management**
+   ```typescript
+   // CapacitorQRScanner.ts
+   private async cleanupResources(): Promise<void> {
+     // Stop scanning
+     await this.stopScan();
+     
+     // Clear any stored camera data
+     this.currentLensFacing = LensFacing.Back;
+     
+     // Remove listeners
+     this.listenerHandles.forEach(handle => handle());
+     this.listenerHandles = [];
+     
+     // Reset state
+     this.isScanning = false;
+     this.updateCameraState('off');
+   }
+   ```
+
+#### Testing on Capacitor
+
+1. **Device Testing**
+   - Test on multiple iOS devices
+   - Test on multiple Android devices
+   - Test different camera configurations
+   - Test with different screen sizes
+   - Test with different OS versions
+
+2. **Camera Testing**
+   - Test front camera switching
+   - Test back camera switching
+   - Test camera permissions
+   - Test camera in use scenarios
+   - Test low light conditions
+   - Test different QR code sizes
+   - Test different QR code distances
+
+3. **Performance Testing**
+   - Battery usage monitoring
+   - Memory usage monitoring
+   - Camera switching speed
+   - QR code detection speed
+   - App responsiveness
+   - Background/foreground transitions