feat: 通用单位系统 — drop/ml/g/capsule，去掉硬编码

unit字段支持4种值，所有显示自动适配：
- drop: 精油(滴/drop)
- ml: 液体产品(ml)
- g: 膏霜产品(g)
- capsule: 胶囊(颗/capsule)
新增产品选单位即可，无需改代码。

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

frontend/src/views/OilReference.vue

@@ -227,7 +227,7 @@
         </div>
         <div class="detail-body">
           <!-- 精油（非ml产品） -->
-          <template v-if="!oils.isMlUnit(selectedOilName)">
+          <template v-if="oils.isDropUnit(selectedOilName)">
             <div class="detail-row">
               <span class="detail-label">总容量</span>
               <span class="detail-value">{{ volumeWithDrops(selectedOilName) }}</span>
@@ -253,19 +253,27 @@
           <template v-else>
             <div class="detail-row">
               <span class="detail-label">总容量</span>
-              <span class="detail-value">{{ getMeta(selectedOilName)?.dropCount || '--' }}ml</span>
+              <span class="detail-value">{{ getMeta(selectedOilName)?.dropCount || '--' }}{{ oils.unitLabel(selectedOilName) }}</span>
             </div>
             <div class="detail-row">
               <span class="detail-label">会员价</span>
               <span class="detail-value">{{ getMeta(selectedOilName)?.bottlePrice != null ? ('¥ ' + getMeta(selectedOilName).bottlePrice.toFixed(2)) : '--' }}</span>
             </div>
             <div class="detail-row">
-              <span class="detail-label">每ml价格</span>
+              <span class="detail-label">每{{ oils.unitLabel(selectedOilName) }}价格</span>
               <span class="detail-value">{{ oils.pricePerDrop(selectedOilName) ? ('¥ ' + oils.pricePerDrop(selectedOilName).toFixed(2)) : '--' }}</span>
             </div>
+            <div class="detail-row" v-if="getMeta(selectedOilName)?.retailPrice">
+              <span class="detail-label">零售价</span>
+              <span class="detail-value">¥ {{ getMeta(selectedOilName).retailPrice.toFixed(2) }}</span>
+            </div>
+            <div class="detail-row" v-if="getMeta(selectedOilName)?.retailPrice && getMeta(selectedOilName)?.dropCount">
+              <span class="detail-label">每{{ oils.unitLabel(selectedOilName) }}价格</span>
+              <span class="detail-value">¥ {{ (getMeta(selectedOilName).retailPrice / getMeta(selectedOilName).dropCount).toFixed(2) }}</span>
+            </div>
           </template>
 
-          <h4 style="margin:16px 0 8px">含此{{ oils.isMlUnit(selectedOilName) ? '产品' : '精油' }}的配方</h4>
+          <h4 style="margin:16px 0 8px">含此{{ oils.isDropUnit(selectedOilName) ? '精油' : '产品' }}的配方</h4>
           <div v-if="recipesWithOil.length" class="detail-recipes">
             <div v-for="r in recipesWithOil" :key="r._id" class="detail-recipe-item">
               <span class="dr-name">{{ r.name }}</span>
@@ -490,27 +498,24 @@ for (const [ml, drops] of Object.entries(VOLUME_OPTIONS)) {
   DROPS_TO_VOLUME[drops] = ml + 'ml'
 }
 
-function volumeWithDrops(name) {
-  const meta = getMeta(name)
-  if (!meta || !meta.dropCount) return '--'
-  if (name === '植物空胶囊') return meta.dropCount + '颗'
-  const ml = DROPS_TO_VOLUME[meta.dropCount]
-  if (ml) return `${ml}/${meta.dropCount}滴`
-  return meta.dropCount + '滴'
-}
-
 function oilPriceUnit(name) {
-  if (oils.isMlUnit(name)) return 'ml'
-  if (name === '植物空胶囊') return '颗'
-  return '滴'
+  return oils.unitLabel(name)
 }
 
 function volumeLabel(dropCount, name) {
-  if (dropCount === 160) return '160颗'
-  if (oils.isMlUnit(name)) return dropCount + 'ml'
+  if (!oils.isDropUnit(name)) return dropCount + oils.unitLabel(name)
   return DROPS_TO_VOLUME[dropCount] || (dropCount + '滴')
 }
 
+function volumeWithDrops(name) {
+  const meta = getMeta(name)
+  if (!meta || !meta.dropCount) return '--'
+  if (!oils.isDropUnit(name)) return meta.dropCount + oils.unitLabel(name)
+  const ml = DROPS_TO_VOLUME[meta.dropCount]
+  if (ml) return `${ml}/${meta.dropCount}滴`
+  return meta.dropCount + '滴'
+}
+
 function chipStyle(name) {
   const hasCard = !!getOilCard(name)
   if (hasCard) return 'cursor:pointer;border-left:3px solid var(--sage);background:linear-gradient(90deg,var(--sage-mist),white)'