Expose scene-authored loading from Environment settings

[untoldengine]

PR summary:

Summary

• Adds a Scene Authored section to the Environment panel
• Restores a visible Load Scene Authored action for Blender-authored cameras and lights
• Reuses the existing scene-authored loading path for .untold assets, tiled scene manifests, and remote streams
• Keeps the Asset Browser context-menu shortcut as a secondary access point