feat(frontend): add styled 404 and error pages

thoughts-frontend/app/error.tsx

@@ -0,0 +1,31 @@
+"use client";
+
+import { useEffect } from "react";
+import { Button } from "@/components/ui/button";
+
+export default function Error({
+  error,
+  reset,
+}: {
+  error: Error & { digest?: string };
+  reset: () => void;
+}) {
+  useEffect(() => {
+    console.error(error);
+  }, [error]);
+
+  return (
+    <div className="min-h-[80vh] flex items-center justify-center px-4">
+      <div className="glass-effect glossy-effect bottom rounded-2xl shadow-fa-lg p-10 text-center max-w-sm w-full">
+        <p className="text-6xl font-black text-muted-foreground/30 mb-4">Oops</p>
+        <h1 className="text-xl font-bold mb-2">Something went wrong</h1>
+        <p className="text-sm text-muted-foreground mb-8">
+          An unexpected error occurred. Try again or come back later.
+        </p>
+        <Button onClick={reset} className="rounded-full">
+          Try again
+        </Button>
+      </div>
+    </div>
+  );
+}

thoughts-frontend/app/not-found.tsx

@@ -0,0 +1,19 @@
+import Link from "next/link";
+import { Button } from "@/components/ui/button";
+
+export default function NotFound() {
+  return (
+    <div className="min-h-[80vh] flex items-center justify-center px-4">
+      <div className="glass-effect glossy-effect bottom rounded-2xl shadow-fa-lg p-10 text-center max-w-sm w-full">
+        <p className="text-6xl font-black text-muted-foreground/30 mb-4">404</p>
+        <h1 className="text-xl font-bold mb-2">Page not found</h1>
+        <p className="text-sm text-muted-foreground mb-8">
+          This page doesn&apos;t exist or was moved.
+        </p>
+        <Button asChild className="rounded-full">
+          <Link href="/">Go home</Link>
+        </Button>
+      </div>
+    </div>
+  );
+}