feat(layout): add admin panel layout with sidebar and header

- Add Sidebar component with navigation items (dashboard, bookings, tournaments, pos, clients, memberships, reports, settings)
- Add Header component with SiteSwitcher and user info/logout
- Add SiteSwitcher component for SUPER_ADMIN multi-site selection
- Add admin layout wrapper with AuthProvider
- Add placeholder Dashboard page

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

apps/web/app/(admin)/dashboard/page.tsx

@@ -0,0 +1,10 @@
+export default function DashboardPage() {
+  return (
+    <div>
+      <h1 className="text-2xl font-bold text-primary-800">Dashboard</h1>
+      <p className="mt-2 text-primary-600">
+        Bienvenido al panel de administración de Padel Pro.
+      </p>
+    </div>
+  );
+}