feat(web-admin): enforce single contract on plan-contract supplement save

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

web-admin/src/views/pl/plan-contract/index.vue

@@ -772,8 +772,12 @@ async function handleSaveStopContracts() {
     ElMessage.warning('请先维护本段客户后再选择合同')
     return
   }
-  if (!supplementDraft.contracts.length) {
-    ElMessage.warning('请先选择至少 1 份合同')
+  if (supplementDraft.contracts.length === 0) {
+    ElMessage.warning('请先选择合同')
+    return
+  }
+  if (supplementDraft.contracts.length > 1) {
+    ElMessage.warning('只能保留一份合同，请移除多余合同后再保存')
     return
   }
   supplementSaving.value = true