Add core components for task management and navigation

This commit introduces the foundational UI components and logic for the task management application, including task creation, calendar views, Kanban boards, and navigation elements.

Replit-Commit-Author: Agent
Replit-Commit-Session-Id: ceced2fc-aa46-458d-ba87-ddd4b7bb1518
Replit-Commit-Checkpoint-Type: full_checkpoint
Replit-Commit-Screenshot-Url: https://storage.googleapis.com/screenshot-production-us-central1/659922a9-0087-461c-90dd-6d9a58b81d4d/ceced2fc-aa46-458d-ba87-ddd4b7bb1518/yy9YLEW

client/src/pages/not-found.tsx

@@ -0,0 +1,21 @@
+import { Card, CardContent } from "@/components/ui/card";
+import { AlertCircle } from "lucide-react";
+
+export default function NotFound() {
+  return (
+    <div className="min-h-screen w-full flex items-center justify-center bg-gray-50">
+      <Card className="w-full max-w-md mx-4">
+        <CardContent className="pt-6">
+          <div className="flex mb-4 gap-2">
+            <AlertCircle className="h-8 w-8 text-red-500" />
+            <h1 className="text-2xl font-bold text-gray-900">404 Page Not Found</h1>
+          </div>
+
+          <p className="mt-4 text-sm text-gray-600">
+            Did you forget to add the page to the router?
+          </p>
+        </CardContent>
+      </Card>
+    </div>
+  );
+}