fix: cert create form credential selection validation

- Explicitly set selectedCredId in onCredSelected
- Separate credential validation from manual input validation

frontend/src/views/CertCreate.vue

@@ -207,6 +207,7 @@ const onProviderChange = () => {
 
 const onCredSelected = (credId: number) => {
   // Just store the selection, backend resolves DNS config
+  selectedCredId.value = credId
 }
 
 const buildDnsConfigJson = (): string => {
@@ -272,7 +273,12 @@ const handleSubmit = async () => {
     }
 
     if (form.challenge_type === 'dns') {
-      if (credMode.value === 'credential' && selectedCredId.value) {
+      if (credMode.value === 'credential') {
+        if (!selectedCredId.value) {
+          ElMessage.warning('请选择一个 DNS 凭证')
+          submitting.value = false
+          return
+        }
         // Use existing credential
         payload.credential_id = selectedCredId.value
       } else if (credMode.value === 'manual') {