New UI mock

For work with no reference screenshot to copy. If there is one, use sp-clone-prototype instead. Measurement beats invention every time.

Everything renders on the canvas from canvases/<slug>/; see sp-canvas for running it, and its references/layout.md for the folder and layout.json rules.

The app ships the template folder, and sp root prints where. sp, refkit or artgen not found? Run sh <sp-canvas skill dir>/scripts/install.sh (Windows: install.ps1) and follow that skill's references/install.md. A [super-prototyping:notice] line on their stderr carries its own rule: finish the step, then do what it says.