Implement dark-first design system with Geist typography

Add Tailwind v4 @theme tokens for surfaces, accents, text, borders,
and status colors. Self-host Geist Sans/Mono variable fonts. Redesign
nav with backdrop blur and active states, home page with gradient hero.
Migrate all 50+ components from ad-hoc gray/blue Tailwind classes to
semantic theme tokens (surface-*, text-*, border-*, accent-*, status-*).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/components/admin/AdminLayout.tsx

@@ -23,7 +23,7 @@ export function AdminLayout() {
                     `block px-3 py-2 rounded-md text-sm font-medium whitespace-nowrap ${
                       isActive
                         ? 'bg-blue-100 text-blue-700 dark:bg-blue-900 dark:text-blue-200'
-                        : 'hover:bg-gray-100 dark:hover:bg-gray-700'
+                        : 'hover:bg-surface-2'
                     }`
                   }
                 >