Add dark mode and theme switching functionality to the application.

Replit-Commit-Author: Agent
Replit-Commit-Session-Id: 705f2157-ef97-4fbd-89e4-8c7f2ecaea90
Replit-Commit-Screenshot-Url: https://storage.googleapis.com/screenshot-production-us-central1/7ed01c5f-a82d-405a-b728-b2e3d127c60c/69154111-0a6b-4eae-ac42-425c5260b653.jpg

client/src/main.tsx

@@ -3,12 +3,15 @@ import { QueryClientProvider } from "@tanstack/react-query";
 import App from "./App";
 import "./index.css";
 import { AuthProvider } from "./hooks/use-auth";
+import { ThemeProvider } from "./hooks/use-theme";
 import { queryClient } from "./lib/queryClient";
 
 createRoot(document.getElementById("root")!).render(
   <QueryClientProvider client={queryClient}>
-    <AuthProvider>
-      <App />
-    </AuthProvider>
+    <ThemeProvider defaultTheme="system" storageKey="ad-management-theme">
+      <AuthProvider>
+        <App />
+      </AuthProvider>
+    </ThemeProvider>
   </QueryClientProvider>
 );