Closes#430.
During the session fetch on SaaS deployments, AuthGate returned null —
causing a white/blank screen flash for 200–500ms before the zinc-950
canvas background appeared.
Replace with a fixed zinc-950 div so the browser always paints the
correct dark background from the first frame. The canvas loading UI
renders on top once the session resolves, with no visible transition.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>