flutter: IMU attitude indicator and UART health check

- WhiskeyMark widget shows roll/pitch as horizon line
- ArduinoData model includes IMU euler angles
- startup waits for Arduino via /health endpoint
- config_service exposes backendUrl

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

pi/ui/lib/services/config_service.dart

@@ -86,4 +86,11 @@ class ConfigService {
     if (value is String && value.isNotEmpty) return value;
     return _defaultNavigator;
   }
+
+  /// Backend URL for API calls
+  String get backendUrl {
+    final value = _config?['backend_url'];
+    if (value is String && value.isNotEmpty) return value;
+    return 'http://127.0.0.1:5000';
+  }
 }