Fix/renderer size

[davidfig]

(This is needed as a prerequisite for the UI header work.)

• [x] rework UI and canvas's HTML and CSS
• [x] canvas properly sizes and resizes with new flow
• [ ] rework pointer events to use pixi events instead of canvas events
  • [x] onPointerDown
  • [ ] onPointerMove
• [ ] ensure code window and other UI works with new flow
• [ ] Keyboard movement properly scrolls when on viewport bounds