fix: add double-submission guard, captcha ARIA labels, and error handling on mount

- Guard onSubmit against concurrent calls via loading flag
- Add role="img" and aria-label to captcha SVG container for accessibility
- Wrap initial captcha fetch in try/catch to show error toast on failure

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

app/pages/register.vue

@@ -34,6 +34,7 @@ async function fetchCaptcha() {
 }
 
 async function onSubmit() {
+  if (loading.value) return
   loading.value = true
   try {
     const res = await $fetch<{ code: number; message: string; data: unknown }>(
@@ -67,7 +68,13 @@ async function onSubmit() {
   }
 }
 
-onMounted(fetchCaptcha)
+onMounted(async () => {
+  try {
+    await fetchCaptcha()
+  } catch {
+    toast.add({ title: '验证码加载失败，请刷新页面', color: 'error' })
+  }
+})
 </script>
 
 <template>
@@ -104,6 +111,8 @@ onMounted(fetchCaptcha)
             <!-- eslint-disable-next-line vue/no-v-html -->
             <div
               class="flex-1 h-10 border rounded-md overflow-hidden bg-[#f8f9fa]"
+              role="img"
+              aria-label="CAPTCHA verification code"
               v-html="captchaSvg"
             />
             <UButton