feat: 非精油产品(drop_count=1)显示"份"而非"滴"

oils store新增 unitLabel/unitLabelPlural，根据 dropCount 判断：
- 精油(dropCount>1): 滴/drop
- 护肤品等(dropCount=1): 份/portion
影响: 配方卡片、个人配方、库存匹配、去重提示

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

frontend/src/stores/oils.js

@@ -93,6 +93,21 @@ export const useOilsStore = defineStore('oils', () => {
     delete oilsMeta.value[name]
   }
 
+  function isPortionUnit(name) {
+    const meta = oilsMeta.value[name]
+    return meta && meta.dropCount === 1
+  }
+
+  function unitLabel(name, lang = 'zh') {
+    if (isPortionUnit(name)) return lang === 'en' ? 'portion' : '份'
+    return lang === 'en' ? 'drop' : '滴'
+  }
+
+  function unitLabelPlural(name, count, lang = 'zh') {
+    if (isPortionUnit(name)) return lang === 'en' ? (count === 1 ? 'portion' : 'portions') : '份'
+    return lang === 'en' ? (count === 1 ? 'drop' : 'drops') : '滴'
+  }
+
   return {
     oils,
     oilsMeta,
@@ -105,5 +120,8 @@ export const useOilsStore = defineStore('oils', () => {
     loadOils,
     saveOil,
     deleteOil,
+    isPortionUnit,
+    unitLabel,
+    unitLabelPlural,
   }
 })