Add runtime BLE permission checks for Android

[unforced]

Problem

The app has BLE permissions declared in AndroidManifest.xml, but doesn't check at runtime if permissions are granted before scanning. This could cause crashes or unexpected behavior.

Current State: Permissions declared ✅ but no runtime checks ❌

Required Permissions (Android)

• BLUETOOTH_SCAN
• BLUETOOTH_CONNECT
• ACCESS_FINE_LOCATION (required for BLE scanning on Android <12)

Suggested Implementation

// Add permission_handler to pubspec.yaml
dependencies:
  permission_handler: ^11.0.0

// In OmiBluetoothService
Future<bool> _checkBlePermissions() async {
  if (!Platform.isAndroid) {
    return true; // iOS handles via Info.plist
  }

  final scanGranted = await Permission.bluetoothScan.isGranted;
  final connectGranted = await Permission.bluetoothConnect.isGranted;
  
  if (!scanGranted || !connectGranted) {
    debugPrint('[OmiBluetoothService] BLE permissions not granted');
    
    // Request permissions
    final status = await [
      Permission.bluetoothScan,
      Permission.bluetoothConnect,
    ].request();
    
    return status[Permission.bluetoothScan]!.isGranted &&
           status[Permission.bluetoothConnect]!.isGranted;
  }
  
  return true;
}

// Call before scanning
Future<void> scanForDevices({...}) async {
  final hasPermissions = await _checkBlePermissions();
  if (!hasPermissions) {
    throw Exception('BLE permissions not granted');
  }
  
  // ... continue with scan
}

Benefits

• Prevents crashes from missing permissions
• Better UX with permission request dialogs
• Follows Android best practices

Priority

High - Required for production Android app

Related

• Part of PR #8 review feedback
• See Android documentation: https://developer.android.com/guide/topics/connectivity/bluetooth/permissions