feat(admin): link city-partner store count; fix benefitUsageRule null string

City partner storeCount jumps to /stores?partnerId=; normalize empty/null text so usage rule is not saved as literal "null.

Co-authored-by: Cursor <cursoragent@cursor.com>

apps/admin-web/src/pages/CityPartnersPage.tsx

@@ -301,7 +301,16 @@ export default function CityPartnersPage() {
       ellipsis: true,
       render: (v) => v || '—',
     },
-    { title: '门店', dataIndex: 'storeCount', width: 60 },
+    {
+      title: '门店',
+      dataIndex: 'storeCount',
+      width: 60,
+      render: (n: number, row) => (
+        <Link to={`/stores?partnerId=${row.id}`} title={`查看「${row.companyName || row.phone}」门店`}>
+          {n ?? 0}
+        </Link>
+      ),
+    },
     { title: '子账号', dataIndex: 'accountCount', width: 70, render: (n) => Math.max(0, n - 1) },
     { title: '创建', dataIndex: 'createdAt', width: 150, render: fmtTime },
     {
@@ -444,7 +453,11 @@ export default function CityPartnersPage() {
           <>
             <Descriptions column={2} size="small" bordered style={{ marginBottom: 16 }}>
               <Descriptions.Item label="城市">{detail.cityName ?? '—'}</Descriptions.Item>
-              <Descriptions.Item label="门店数">{detail.storeCount ?? 0}</Descriptions.Item>
+              <Descriptions.Item label="门店数">
+                <Link to={`/stores?partnerId=${detail.id}`} title="查看该城市合伙人门店">
+                  {detail.storeCount ?? 0}
+                </Link>
+              </Descriptions.Item>
               <Descriptions.Item label="管仓仓库" span={2}>
                 {detail.managedWarehouseName ?? (
                   <Typography.Text type="secondary">