fix: dashboard updates when switching sites

- Added SiteContext for global site selection state
- Updated admin layout with SiteProvider
- Updated SiteSwitcher to use shared context
- Dashboard now refetches data when site changes

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

apps/web/app/(admin)/layout.tsx

@@ -1,4 +1,5 @@
 import { AuthProvider } from '@/components/providers/auth-provider';
+import { SiteProvider } from '@/contexts/site-context';
 import { Sidebar } from '@/components/layout/sidebar';
 import { Header } from '@/components/layout/header';
 
@@ -9,13 +10,15 @@ export default function AdminLayout({
 }) {
   return (
     <AuthProvider>
-      <div className="min-h-screen bg-primary-50">
-        <Sidebar />
-        <div className="pl-64">
-          <Header />
-          <main className="p-6">{children}</main>
+      <SiteProvider>
+        <div className="min-h-screen bg-primary-50">
+          <Sidebar />
+          <div className="pl-64">
+            <Header />
+            <main className="p-6">{children}</main>
+          </div>
         </div>
-      </div>
+      </SiteProvider>
     </AuthProvider>
   );
 }