* feat(admin): add responsive mobile sidebar drawer The admin layout had a fixed 256px sidebar with no responsive breakpoints, leaving only ~119px for content on mobile viewports. Now the sidebar is hidden below md (768px) and replaced with a hamburger button that opens a Radix Dialog-based slide-in drawer. - Extract shared AdminNav component for desktop sidebar and mobile drawer - Add hamburger menu button (md:hidden) with sticky top bar - Slide-in drawer with overlay, close button, and auto-close on link click - Proper a11y: dialog role, sr-only title, focus management via Radix - Add slide/fade animation keyframes to globals.css - Add admin sub-routes to mobile-audit e2e test coverage - 6 new unit tests for drawer open/close/a11y behavior * style(admin): fix Prettier formatting in admin layout