feat(ui): kumo-style app shell for the authenticated area

[matthew-demidoff]

Summary

Rebuilds the logged-in layout to match a clean dashboard reference. The previous authed shell (full-height sidebar, header only over the content) is replaced by a shared AppShell:

• Full-width top header (58px): brand + breadcrumb left; docs / sign out (and an Admin link) right.
• Collapsible icon sidebar (260px ↔ 57px), persisted to localStorage, with a 250ms cubic-bezier width animation, grouped nav with icons, and ⌘K search.
• Centered max-w-1400 content with a staggered entrance.
• Sections become elevated-header cards (hairline ring + shadow-xs, h-14 header).

Wired onto it: the dashboard, security center, request-bearer, all developer pages, and the admin area (an admin variant with admin nav + a red Admin badge). The old Sidebar/AdminSidebar are removed. Adds lucide-react (MIT, tree-shakeable) for nav/utility icons.

Verification

• tsc clean, next build green (50 routes), Playwright 12/12, unit suite green.
• Screenshotted both the user and admin shell variants (expanded + collapsed sidebar).

No server, API, or schema changes.