Replace floating pill header with static variant for authenticated
views. Add collapsible sidebar with site picker, grouped navigation
(Analytics/Infrastructure), and mobile overlay drawer. Remove
horizontal SiteNav tab bar.
SiteNav now lives in the [id] layout instead of each page, so it stays
mounted during route transitions. Switching between Dashboard, Uptime,
Funnels, and Settings no longer flashes a full-page skeleton.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
