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/components/theme-toggle.tsx

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+import * as React from "react";
+import { Moon, Sun, Laptop } from "lucide-react";
+import { Button } from "@/components/ui/button";
+import {
+  DropdownMenu,
+  DropdownMenuContent,
+  DropdownMenuItem,
+  DropdownMenuTrigger,
+} from "@/components/ui/dropdown-menu";
+import { useTheme } from "@/hooks/use-theme";
+
+export function ThemeToggle() {
+  const { theme, setTheme } = useTheme();
+
+  return (
+    <DropdownMenu>
+      <DropdownMenuTrigger asChild>
+        <Button variant="ghost" size="icon" className="relative h-9 w-9 rounded-md">
+          <Sun className="h-5 w-5 rotate-0 scale-100 transition-all dark:-rotate-90 dark:scale-0" />
+          <Moon className="absolute h-5 w-5 rotate-90 scale-0 transition-all dark:rotate-0 dark:scale-100" />
+          <span className="sr-only">Toggle theme</span>
+        </Button>
+      </DropdownMenuTrigger>
+      <DropdownMenuContent align="end">
+        <DropdownMenuItem onClick={() => setTheme("light")}>
+          <Sun className="mr-2 h-4 w-4" />
+          <span>Light</span>
+        </DropdownMenuItem>
+        <DropdownMenuItem onClick={() => setTheme("dark")}>
+          <Moon className="mr-2 h-4 w-4" />
+          <span>Dark</span>
+        </DropdownMenuItem>
+        <DropdownMenuItem onClick={() => setTheme("system")}>
+          <Laptop className="mr-2 h-4 w-4" />
+          <span>System</span>
+        </DropdownMenuItem>
+      </DropdownMenuContent>
+    </DropdownMenu>
+  );
+}