fix: 未登录用户每次提示上传QR，已登录每月一次

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/components/RecipeDetailOverlay.vue

@@ -516,12 +516,19 @@ async function loadBrand() {
   } catch {
     brand.value = {}
   }
-  // Prompt QR upload at most once per month
+  // Prompt QR upload: logged-in users once per month, anonymous every time
   if (showBrandHint.value) {
-    const lastPrompt = localStorage.getItem('qr_upload_prompt_time')
-    const oneMonth = 30 * 24 * 60 * 60 * 1000
-    if (!lastPrompt || Date.now() - Number(lastPrompt) > oneMonth) {
-      localStorage.setItem('qr_upload_prompt_time', String(Date.now()))
+    let shouldPrompt = true
+    if (authStore.isLoggedIn) {
+      const lastPrompt = localStorage.getItem('qr_upload_prompt_time')
+      const oneMonth = 30 * 24 * 60 * 60 * 1000
+      if (lastPrompt && Date.now() - Number(lastPrompt) < oneMonth) {
+        shouldPrompt = false
+      } else {
+        localStorage.setItem('qr_upload_prompt_time', String(Date.now()))
+      }
+    }
+    if (shouldPrompt) {
       const ok = await showConfirm(
         '上传你的专属二维码，让配方卡片更专业 ✨',
         { okText: '去上传', cancelText: '下次再说' }