Refactor project structure and enhance functionality

Changed the project namespace from `Ghost.Editor` to `Ghost.App` across multiple files.
Changed the `InternalsVisibleTo` attribute in `AssemblyInfo.cs` to include `Ghost.App`.
Changed the `ProjectRepository` class to add new asynchronous methods for retrieving projects by ID, name, and metadata path.
Changed the `ProjectService` class to utilize the new asynchronous project loading methods.
Changed the `SceneGraph` classes to improve node management and serialization.
Changed the `EntityManager` class to enhance entity management with new component handling methods.
Added new test classes, `EntityTest` and `SerializationTest`, to ensure reliability in entity and serialization systems.
Added the `Ghost.App` project file to establish a modular project structure.
Added the `Ghost.Generator` project for automated component serialization code generation.
Updated UI components to reflect the new namespace for proper functionality.

Ghost.Editor/SceneGraph/SceneGraphNode.cs

@@ -0,0 +1,54 @@
+using CommunityToolkit.Mvvm.ComponentModel;
+using System.Collections.ObjectModel;
+
+namespace Ghost.Editor.SceneGraph;
+
+public enum SceneGraphNodeType
+{
+    Scene,
+    Entity,
+}
+
+public abstract partial class SceneGraphNode : ObservableObject
+{
+    public ObservableCollection<SceneGraphNode>? Children
+    {
+        get;
+        private set;
+    }
+
+    [ObservableProperty]
+    public partial string Name
+    {
+        get;
+        set;
+    }
+
+    public abstract SceneGraphNodeType NodeType
+    {
+        get;
+    }
+
+    public int ChildCount => Children?.Count ?? 0;
+
+    public virtual void AddChild(SceneGraphNode child)
+    {
+        Children ??= new();
+        Children.Add(child);
+    }
+
+    public virtual bool RemoveChild(SceneGraphNode child)
+    {
+        return Children?.Remove(child) ?? false;
+    }
+
+    public SceneGraphNode GetChild(int index)
+    {
+        if (Children == null || index < 0 || index >= Children.Count)
+        {
+            throw new ArgumentOutOfRangeException(nameof(index), "Index is out of range.");
+        }
+
+        return Children[index];
+    }
+}