enhance auth modals UX and add download login check

apps/jingrow/frontend/src/locales/zh-CN.json

@@ -1154,6 +1154,7 @@
   "Network error. Please check your connection.": "网络错误，请检查您的连接",
   "No image data returned": "未返回图片数据",
   "Please login first to use this feature": "请先登录以使用此功能",
+  "Please login to download": "请先登录后才能下载",
   "Session expired. Please login again.": "会话已过期，请重新登录",
   "Session expired. Please login again to use this feature.": "会话已过期，请重新登录以使用此功能",
   "or": "或",

apps/jingrow/frontend/src/views/HomePage.vue

@@ -383,6 +383,13 @@ const handleRemoveBackground = async () => {
 const handleDownload = async () => {
   if (!resultImage.value) return
 
+  // 检查登录状态
+  if (!isLoggedIn.value) {
+    message.warning(t('Please login to download'))
+    showLoginModal.value = true
+    return
+  }
+
   try {
     const response = await fetch(resultImage.value)
     const blob = await response.blob()