editor camera: dedicated Scene-view camera decoupled from camera entities

EditorCamera (UE FEditorViewportClient semantics) owned by SceneRenderer:
own TransformComponent + reused Camera controller. BuildFrame dispatches
render source by display mode — editor form always renders from the
editor camera (usable in empty scenes), game form renders from the
scene's primary camera entity. AlignTo copies an entity pose into the
editor camera ("Align Viewport to Camera" button in Inspector). F focus
and viewport gizmo now follow the same dispatch. main.cpp stripped of
dangling sandbox camera/spinner state; onSceneLoaded hook removed.

App/editor/Editor.cpp

@@ -367,10 +367,4 @@ void Editor::DoLoadScene()
                         "keeping current", envPath);
         }
     }
-
-    // 应用层重解析悬空句柄（相机等沙盒状态）
-    if (m_Ctx.onSceneLoaded)
-    {
-        m_Ctx.onSceneLoaded();
-    }
 }