feat(design-system): add not-found, error, loading pages

web/src/app/error.tsx

@@ -0,0 +1,25 @@
+'use client';
+
+import { useEffect } from 'react';
+
+export default function Error({ error, reset }: { error: Error & { digest?: string }; reset: () => void }) {
+  useEffect(() => { console.error('Unhandled error:', error); }, [error]);
+
+  return (
+    <div className="flex min-h-screen items-center justify-center p-4" style={{ background: 'var(--nl-bg-canvas, #06070A)' }}>
+      <div className="text-center">
+        <h1 className="text-4xl font-bold" style={{ color: 'var(--nl-danger, #FF6E6E)' }}>Something went wrong</h1>
+        <p className="mt-4 text-sm" style={{ color: 'var(--nl-text-secondary, #A5B1C7)' }}>
+          {error.message || 'An unexpected error occurred.'}
+        </p>
+        <button
+          onClick={reset}
+          className="mt-6 inline-flex items-center gap-2 rounded-lg px-4 py-2 text-sm font-medium transition-colors"
+          style={{ background: 'var(--nl-accent-primary, #5A8CFF)', color: '#fff' }}
+        >
+          Try again
+        </button>
+      </div>
+    </div>
+  );
+}

web/src/app/loading.tsx

@@ -0,0 +1,13 @@
+export default function Loading() {
+  return (
+    <div className="flex min-h-screen items-center justify-center" style={{ background: 'var(--nl-bg-canvas, #06070A)' }}>
+      <div className="flex flex-col items-center gap-3">
+        <div
+          className="h-8 w-8 animate-spin rounded-full border-2 border-t-transparent"
+          style={{ borderColor: 'var(--nl-accent-primary, #5A8CFF)', borderTopColor: 'transparent' }}
+        />
+        <p className="text-sm" style={{ color: 'var(--nl-text-secondary, #A5B1C7)' }}>Loading...</p>
+      </div>
+    </div>
+  );
+}