fix: QR上传提醒改为每月一次，按钮始终显示

- 用 localStorage 记录上次弹窗时间，30天内不再弹
- 📲 上传二维码按钮始终显示（只要用户没上传QR）
- 弹窗取消按钮文案改为「下次再说」

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

frontend/src/components/RecipeDetailOverlay.vue

@@ -516,14 +516,19 @@ async function loadBrand() {
   } catch {
     brand.value = {}
   }
-  // Show upload prompt if user hasn't set up brand assets yet
+  // Prompt QR upload at most once per month
   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()))
       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: '取消' }
+        { okText: '去上传', cancelText: '下次再说' }
       )
       if (ok) goUploadQr()
     }
+  }
 }
 
 // Whether to show the brand/QR upload hint (show to all users who haven't set up brand assets)