feat: add customer cart with expiry, shipping addresses with USPS validation, and multi-item PayPal checkout

frontend/src/admin/Settings.tsx

@@ -0,0 +1,38 @@
+import { useEffect, useState } from 'react';
+import { Form, InputNumber, Button, Typography, message, Card } from 'antd';
+import { fetchAdminSettings, updateAdminSettings } from './adminSettingsApi';
+
+const { Title, Text } = Typography;
+
+export default function Settings() {
+  const [form] = Form.useForm();
+  const [loading, setLoading] = useState(true);
+
+  useEffect(() => {
+    fetchAdminSettings().then(s => {
+      form.setFieldsValue({ cartExpiryHours: s.cartExpiryHours });
+      setLoading(false);
+    });
+  }, [form]);
+
+  async function handleSave() {
+    const values = await form.validateFields();
+    await updateAdminSettings(values);
+    message.success('Settings saved');
+  }
+
+  return (
+    <Card style={{ maxWidth: 480 }}>
+      <Title level={4}>Cart Settings</Title>
+      <Text type="secondary">
+        How long an item stays reserved in a customer's cart before it's automatically released back to available inventory.
+      </Text>
+      <Form form={form} layout="vertical" style={{ marginTop: 16 }} disabled={loading}>
+        <Form.Item name="cartExpiryHours" label="Cart expiry (hours)" rules={[{ required: true }]}>
+          <InputNumber min={0.5} step={0.5} style={{ width: '100%' }} />
+        </Form.Item>
+        <Button type="primary" onClick={handleSave} loading={loading}>Save</Button>
+      </Form>
+    </Card>
+  );
+}