fix(ui): responsive primary nav so phones aren't stuck on one page

The AppShell side nav was a hard `permanent` 292px v-navigation-drawer with no
toggle. On a phone it left almost no content width and the section links got
covered by in-page content, so users got stuck on whatever page they were on
(reported on the Catalog page) with no way to switch sections.

Make it the standard responsive drawer: permanent on >= md, and a temporary
overlay toggled by a new v-app-bar-nav-icon on smAndDown. Drawer starts closed on
mobile and auto-closes after navigating. Verified headless at 375px (hamburger
shows, drawer opens/closes with scrim) and 1400px (permanent, no hamburger).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
