setup.html was the only page not using the .app-shell CSS grid — its
elements were directly on the body with inline position styles. Wraps
content in .app-shell--live (viewport-filling variant), moves header
into .app-header grid area, and nests #scene-container inside .app-main.
Verified: grep for position:absolute across dashboard/css/ returns only
intentional decorative/component uses (pseudo-elements, toggles, tooltips,
dropdowns, canvas overlays) — zero on layout containers.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
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