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)/layout.tsx

@@ -0,0 +1,21 @@
+import { AuthProvider } from '@/components/providers/auth-provider';
+import { Sidebar } from '@/components/layout/sidebar';
+import { Header } from '@/components/layout/header';
+
+export default function AdminLayout({
+  children,
+}: {
+  children: React.ReactNode;
+}) {
+  return (
+    <AuthProvider>
+      <div className="min-h-screen bg-primary-50">
+        <Sidebar />
+        <div className="pl-64">
+          <Header />
+          <main className="p-6">{children}</main>
+        </div>
+      </div>
+    </AuthProvider>
+  );
+}