Adds a Preview button next to Radio Pascal that switches the editor
into a viewer-like experience without leaving the page. The preview
mode swaps in the viewer's selection manager (hierarchical drill-down),
zone system, camera controls (left-click pan, auto-navigate), and
interactive item system while hiding editor-only UI (tools, panels,
sidebar, grid). Back arrow in the viewer overlay returns to the editor.
Also widens the default sidebar from 288px to 432px for better
project title visibility.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
