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/mini-user/src/pages/store-detail/index.tsx

@@ -174,7 +174,9 @@ export default function StoreDetailPage() {
   ]);
 
   const intro = store.intro?.trim() || '';
-  const benefitRule = store.benefitUsageRule?.trim() || '';
+  const benefitRuleRaw = store.benefitUsageRule?.trim() || '';
+  const benefitRule =
+    benefitRuleRaw && !/^null$/i.test(benefitRuleRaw) ? benefitRuleRaw : '';
 
   function previewEnv(index: number) {
     if (!envPhotos.length) return;