Fix model substitution and clean mesh FBX generation logic to ensure GLB compatibility

Prevent Blender from overwriting raw Trellis GLB file

Fix glTF binary corruption in Blender export scripts

Use non-destructive fast cleanup for FBX export to preserve original UV mapping, materials, and textures without distortion

Fix EXT_texture_webp GLB import error in Blender by upgrading to Blender 4.2 LTS and sanitizing glTF extension headers

Restructure repository layout: move server and scripts to backend/ and web assets to frontend/ (ignore database and generated outputs)