Gate renderer/custom-defines fields behind Advanced options

[danoli3]

Summary

• Custom defines and the Renderer fields (from Add custom preprocessor defines and a macOS/iOS renderer selector #651) were always visible regardless of Advanced options - now gated the same way as the other advanced-only fields (#sourceExtraSection, #emsdkField, etc)
• Relabel the renderer dropdown to describe what each choice actually does instead of just naming the library: ANGLE - Auto OpenGL to Metal, Dawn - Auto OpenGL to WebGPU/Metal, Default (OpenGL / OpenGL ES)

Notes

• Labels assume the Metal backend since the field is still osx/ios/macos-only (unchanged scope) - Linux/Windows aren't wired up to ANGLE-Vulkan or Dawn-Vulkan packages yet, so left out for now rather than showing an option that doesn't build

Test plan

• Verified live (real Electron launch): Advanced off hides both fields, Advanced on + osx selected shows both, toggling advanced back off hides the renderer field again
• No console/page errors