Add XR controls to UI (VR/AR start/stop, AR overlay) (#558)

[DhanashreePetare]

Description

• Add an XR section to both dat.GUI and Phoenix menu with Enter VR, Enter AR, AR DOM overlay toggle, and Exit XR actions.
• Wire XR controls into UI initialization; AR overlay flag is configurable.
• Update XR guide with how to launch XR from the UI and add changelog entry.

Testing

1. yarn install (succeeds with peer warnings)
2. yarn start / yarn start:ssl and manually verified XR controls appear and invoke actions.

Result