Gesture bar home button behaviour on iOS

[tracygardner]

Virtual gesture bar and AssistiveTouch virtual home button: Review UX and adapt as needed. Make sure UI doesn't interfere with OS top and bottom bar on mobile and follow best practices.

• Review on a range of devices and identify issues in both browser fullscreen and home screen app mode @lawsie
• Stop Flock XR chrome being over OS chrome on iPad
• Improve UX affordance in bottom bar for switching between code and canvas (see NLnet UX report for recommendations) @lawsie
• Top bar in fullscreen on iPad appearing over the top bar with clock etc
• Recover from webgl context loss rather than error banner

[lawsie]

#662
Added pill toggle button to switch between canvas and code (as per Miroslav's suggestions)

[lawsie]

Tested on iPhone 14, iPad 6th gen, Samsung Galaxy Tab A9 and various issues raised and fixed by #665 #666 #667