feat: 护肤品用ml单位，精油用滴，通过unit字段区分

- 后端: oils表新增unit列(drop/ml)，API返回unit字段
- 前端: 根据unit='ml'显示ml单位，精油显示滴/drop
- 护肤品drop_count改为实际ml容量，成本按用量比例计算
  如玫瑰护手霜100ml ¥135，配方用30ml → 成本¥40.5

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

frontend/src/stores/oils.js

@@ -70,6 +70,7 @@ export const useOilsStore = defineStore('oils', () => {
         retailPrice: oil.retail_price ?? null,
         isActive: oil.is_active !== 0,
         enName: oil.en_name ?? null,
+        unit: oil.unit || 'drop',
       }
     }
     oils.value = newOils
@@ -93,18 +94,22 @@ export const useOilsStore = defineStore('oils', () => {
     delete oilsMeta.value[name]
   }
 
-  function isPortionUnit(name) {
+  function isMlUnit(name) {
     const meta = oilsMeta.value[name]
-    return meta && meta.dropCount === 1
+    return meta && meta.unit === 'ml'
+  }
+
+  function isPortionUnit(name) {
+    return isMlUnit(name)
   }
 
   function unitLabel(name, lang = 'zh') {
-    if (isPortionUnit(name)) return lang === 'en' ? 'portion' : '份'
+    if (isMlUnit(name)) return 'ml'
     return lang === 'en' ? 'drop' : '滴'
   }
 
   function unitLabelPlural(name, count, lang = 'zh') {
-    if (isPortionUnit(name)) return lang === 'en' ? (count === 1 ? 'portion' : 'portions') : '份'
+    if (isMlUnit(name)) return 'ml'
     return lang === 'en' ? (count === 1 ? 'drop' : 'drops') : '滴'
   }