fix: 卡片预览加入容量切换，非单次滴数四舍五入

- VOLUME_DROPS 新增「单次」(null，不缩放)
- 新增 scaleIngredients() 按比例缩放成分
- 卡片预览区加入容量切换按钮，切换后实时更新滴数与成本
- priceInfo 与 coconutDrops 均基于缩放后数据

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

frontend/src/components/RecipeDetailOverlay.vue

@@ -36,6 +36,17 @@
           >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>
@@ -382,6 +393,7 @@ 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({})
@@ -400,14 +412,30 @@ const canEditThisRecipe = computed(() => {
 
 const isFav = computed(() => recipesStore.isFavorite(recipe.value))
 
-// Card ingredients: exclude coconut oil
+// Scale ingredients proportionally to target volume; '单次' = no scaling
-const cardIngredients = computed(() =>
+function scaleIngredients(ingredients, volume) {
-  recipe.value.ingredients.filter(ing => ing.oil !== '椰子油')
+  const targetDrops = VOLUME_DROPS[volume]
+  if (!targetDrops) return ingredients  // 单次：不缩放
+  const totalDrops = ingredients.reduce((sum, ing) => sum + (ing.drops || 0), 0)
+  if (totalDrops === 0) return ingredients
+  return ingredients.map(ing => ({
+    ...ing,
+    drops: Math.round(ing.drops * targetDrops / totalDrops),
+  }))
+}
+
+// Card ingredients: scaled to selected volume, coconut oil excluded from display
+const scaledCardIngredients = computed(() =>
+  scaleIngredients(recipe.value.ingredients, selectedCardVolume.value)
 )
 
-// Coconut oil drops
+const cardIngredients = computed(() =>
+  scaledCardIngredients.value.filter(ing => ing.oil !== '椰子油')
+)
+
+// Coconut oil drops (from scaled set)
 const coconutDrops = computed(() => {
-  const coco = recipe.value.ingredients.find(ing => ing.oil === '椰子油')
+  const coco = scaledCardIngredients.value.find(ing => ing.oil === '椰子油')
   return coco ? coco.drops : 0
 })
 
@@ -433,7 +461,7 @@ const dilutionDesc = computed(() => {
     : `该配方适用于单次用量（共${totalDrops}滴），其中纯精油 ${totalEoDrops.value} 滴，椰子油 ${coconutDrops.value} 滴，稀释比例为 1:${ratio}`
 })
 
-const priceInfo = computed(() => oilsStore.fmtCostWithRetail(recipe.value.ingredients))
+const priceInfo = computed(() => oilsStore.fmtCostWithRetail(scaledCardIngredients.value))
 
 // Today string
 const todayStr = computed(() => {
@@ -512,6 +540,12 @@ function switchLang(lang) {
   nextTick(() => generateCardImage())
 }
 
+function onCardVolumeChange(ml) {
+  selectedCardVolume.value = ml
+  cardImageUrl.value = null
+  nextTick(() => generateCardImage())
+}
+
 async function saveImage() {
   if (!cardImageUrl.value) {
     await generateCardImage()
@@ -1469,6 +1503,10 @@ async function saveRecipe() {
   margin-bottom: 10px;
 }
 
+.card-volume-controls {
+  margin: 8px 0 12px;
+}
+
 .volume-btn {
   padding: 7px 16px;
   border: 1.5px solid var(--border, #e0d4c0);

frontend/src/stores/oils.js

@@ -5,6 +5,7 @@ import { api } from '../composables/useApi'
 export const DROPS_PER_ML = 18.6
 
 export const VOLUME_DROPS = {
+  '单次': null,
   '2.5': 46,
   '5': 93,
   '10': 186,