[Fix] GSplatComponent.customAabb to fall back to resource AABB

[mvaligursky]

Fix GSplatComponent.customAabb to fall back to resource AABB

Summary

• Fixes customAabb getter to return the resource's AABB when no custom AABB is explicitly set
• Restores backwards compatibility broken by the AABB refactoring in Add GSplat procedural shape scripts and example, other fixes and imrovements #8415
• Fixes the viewer example failing to orbit when loading PLY files via drag-and-drop

Technical Details

The customAabb getter now falls back to:

1. placement.aabb in unified mode (which itself falls back to resource.aabb)
2. resource.aabb in non-unified mode