Introduce background spawning with parallax effect (#86)

Co-authored-by: Michal Pikulski <michal.a.pikulski@gmail.com>
Reviewed-on: #86

Assets/Editor/Minigames/Airplane/GroundDistanceSpawnerEditor.cs

@@ -0,0 +1,63 @@
+using Minigames.Airplane.Core.Spawning;
+using UnityEditor;
+using UnityEngine;
+
+namespace Editor.Minigames.Airplane
+{
+    /// <summary>
+    /// Simplified custom editor for GroundDistanceSpawner.
+    /// Shows only ground-relevant fields. Ground Y and interval are in AirplaneSettings.
+    /// </summary>
+    [CustomEditor(typeof(GroundDistanceSpawner))]
+    public class GroundDistanceSpawnerEditor : UnityEditor.Editor
+    {
+        private SerializedProperty _poolsProperty;
+        
+        private void OnEnable()
+        {
+            _poolsProperty = serializedObject.FindProperty("pools");
+            
+            // Ensure exactly 1 pool exists
+            if (_poolsProperty.arraySize != 1)
+            {
+                _poolsProperty.arraySize = 1;
+                serializedObject.ApplyModifiedProperties();
+            }
+        }
+        
+        public override void OnInspectorGUI()
+        {
+            serializedObject.Update();
+            
+            // Ensure exactly 1 pool
+            if (_poolsProperty.arraySize != 1)
+            {
+                _poolsProperty.arraySize = 1;
+            }
+            
+            // Draw single pool
+            EditorGUILayout.LabelField("Ground Tiles (Fixed: 1 pool)", EditorStyles.boldLabel);
+            
+            var poolElement = _poolsProperty.GetArrayElementAtIndex(0);
+            var prefabsProperty = poolElement.FindPropertyRelative("prefabs");
+            
+            EditorGUILayout.BeginVertical(EditorStyles.helpBox);
+
+            EditorGUILayout.PropertyField(prefabsProperty, new GUIContent("Prefabs"), true);
+            
+            if (prefabsProperty.arraySize == 0)
+            {
+                EditorGUILayout.HelpBox("Add at least one ground tile prefab", MessageType.Warning);
+            }
+            else
+            {
+                EditorGUILayout.LabelField($"Prefabs: {prefabsProperty.arraySize}", EditorStyles.miniLabel);
+            }
+            
+            EditorGUILayout.EndVertical();
+
+            serializedObject.ApplyModifiedProperties();
+        }
+    }
+}
+