feat: implement authentication UI components

- Add LoginForm component with email/password validation
- Add RegisterForm component with password confirmation
- Create login page at /auth/login
- Create register page at /auth/register
- Create authentication error page at /auth/error
- Add loading states and error handling
- Implement inline validation with user feedback
- Follow existing component patterns from ListForm

Implements task 6 from user-auth-and-sharing spec
Requirements: 1.1, 2.1

app/auth/register/page.tsx

@@ -0,0 +1,53 @@
+import Link from 'next/link'
+import RegisterForm from '@/components/RegisterForm'
+import { Suspense } from 'react'
+
+export const metadata = {
+  title: 'Register - Ultimate Lists',
+  description: 'Create your Ultimate Lists account',
+}
+
+function RegisterContent() {
+  return (
+    <div className="min-h-screen flex items-center justify-center bg-gray-50 px-4 py-12">
+      <div className="max-w-md w-full space-y-8">
+        <div className="text-center">
+          <h1 className="text-3xl sm:text-4xl font-bold text-gray-900 mb-2">
+            Create Account
+          </h1>
+          <p className="text-sm sm:text-base text-gray-600">
+            Sign up to start organizing your tasks
+          </p>
+        </div>
+
+        <div className="bg-white rounded-xl shadow-md p-6 sm:p-8">
+          <RegisterForm />
+
+          <div className="mt-6 text-center">
+            <p className="text-sm text-gray-600">
+              Already have an account?{' '}
+              <Link
+                href="/auth/login"
+                className="font-medium text-blue-600 hover:text-blue-500 transition-colors"
+              >
+                Log in
+              </Link>
+            </p>
+          </div>
+        </div>
+      </div>
+    </div>
+  )
+}
+
+export default function RegisterPage() {
+  return (
+    <Suspense fallback={
+      <div className="min-h-screen flex items-center justify-center bg-gray-50">
+        <div className="text-gray-600">Loading...</div>
+      </div>
+    }>
+      <RegisterContent />
+    </Suspense>
+  )
+}