feat: 未登录用户也显示二维码上传提示，点击时引导登录/注册

- RecipeDetailOverlay: 未上传二维码/背景图时，卡片上方显示提示横幅，下方出现「上传我的二维码」按钮，点击跳转到 MyDiary 品牌设置页并记录来源配方
- MyDiary: 新增二维码图片上传区域（直接上传图片文件，存为 base64 → PUT /api/brand qr_code 字段）；上传成功后若有待返回配方则自动跳回配方卡片；修复 loadBrandSettings 字段名与后端不一致的问题
- RecipeSearch: 支持 ?openRecipe= 查询参数，页面挂载时自动打开指定配方卡片，实现从 MyDiary 上传后无缝返回

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

frontend/src/components/LoginModal.vue

@@ -119,7 +119,7 @@ async function submit() {
   position: fixed;
   inset: 0;
   background: rgba(0, 0, 0, 0.35);
-  z-index: 6000;
+  z-index: 5000;
   display: flex;
   align-items: center;
   justify-content: center;

frontend/src/components/RecipeDetailOverlay.vue

@@ -36,17 +36,6 @@
           >English</button>
         </div>
 
-        <!-- Volume selector -->
-        <div class="volume-controls card-volume-controls">
-          <button
-            v-for="(drops, ml) in VOLUME_DROPS"
-            :key="ml"
-            class="volume-btn"
-            :class="{ active: selectedCardVolume === ml }"
-            @click="onCardVolumeChange(ml)"
-          >{{ ml === '单次' ? '单次' : ml + 'ml' }}</button>
-        </div>
-
         <!-- QR / brand upload hint -->
         <div v-if="showBrandHint" class="brand-upload-hint">
           <span class="hint-icon">✨</span>
@@ -393,7 +382,6 @@ const viewMode = ref('card')
 const cardRef = ref(null)
 const cardImageUrl = ref(null)
 const cardLang = ref('zh')
-const selectedCardVolume = ref('单次')
 const showTranslationEditor = ref(false)
 const customRecipeNameEn = ref('')
 const customOilNameEn = ref({})