QR upload: replace inline hint with dialog, add back button in brand page

- Replace the inline brand-upload-hint bar in RecipeDetailOverlay with a
  confirm dialog (「去上传」/「取消」) that pops up when the card opens
- Extend useDialog/CustomDialog to support custom ok/cancel button text
- Add a 「← 返回配方卡片」banner in MyDiary brand tab when navigated
  from a recipe card, allowing return without uploading

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/components/RecipeDetailOverlay.vue

@@ -36,12 +36,6 @@
           >English</button>
         </div>
 
-        <!-- QR / brand upload hint -->
-        <div v-if="showBrandHint" class="brand-upload-hint">
-          <span class="hint-icon">✨</span>
-          <span>上传你的专属二维码，生成属于自己的配方卡片</span>
-        </div>
-
         <!-- Card image (rendered by html2canvas) -->
         <div v-show="!cardImageUrl" ref="cardRef" class="export-card">
           <!-- Brand overlay layers -->
@@ -361,6 +355,7 @@ import { useAuthStore } from '../stores/auth'
 import { useUiStore } from '../stores/ui'
 import { useDiaryStore } from '../stores/diary'
 import { api } from '../composables/useApi'
+import { showConfirm } from '../composables/useDialog'
 import { oilEn, recipeNameEn } from '../composables/useOilTranslation'
 // TagPicker replaced with inline tag editing
 
@@ -481,6 +476,14 @@ async function loadBrand() {
   } catch {
     brand.value = {}
   }
+  // Show upload prompt if user hasn't set up brand assets yet
+  if (showBrandHint.value) {
+    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' }
+    )
+    if (ok) goUploadQr()
+  }
 }
 
 // Whether to show the brand/QR upload hint (show to all users who haven't set up brand assets)