feat: 配方volume字段存储编辑器选择的容量

- 后端: recipes表新增volume列，API返回/保存volume
- 前端: 保存时发送formVolume，编辑时优先用stored volume
- 容量显示优先级: stored volume > 椰子油计算 > 产品ml求和
- 修复编辑器容量选择保存后不生效的bug

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

frontend/src/components/RecipeCard.vue

@@ -48,6 +48,15 @@ const priceInfo = computed(() => oilsStore.fmtCostWithRetail(props.recipe.ingred
 const isFav = computed(() => recipesStore.isFavorite(props.recipe))
 
 const volumeLabel = computed(() => {
+  // Priority 1: stored volume from editor selection
+  const vol = props.recipe.volume
+  if (vol) {
+    if (vol === 'single') return '单次'
+    if (vol === 'custom') return ''
+    if (/^\d+$/.test(vol)) return `${vol}ml`
+    return vol
+  }
+  // Priority 2: calculate from ingredients
   const ings = props.recipe.ingredients || []
   const coco = ings.find(i => i.oil === '椰子油')
   if (coco && coco.drops) {
@@ -56,7 +65,7 @@ const volumeLabel = computed(() => {
     if (ml <= 2) return '单次'
     return `${Math.round(ml)}ml`
   }
-  // Non-coconut: sum all portion products as ml
+  // Priority 3: sum portion products as ml
   let totalMl = 0
   let hasProduct = false
   for (const ing of ings) {