BackgroundScene hosts the scene behind the app (fixed, z-index -1, tier off
renders nothing); mountBackgroundScene owns the async start/dispose race and
calls login() when a session already exists. Signed-out screens go transparent
while a scene runs. build:preview sets backgroundScene.tier low in dist.
Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
