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/stores/oils.js

@@ -94,25 +94,39 @@ export const useOilsStore = defineStore('oils', () => {
     delete oilsMeta.value[name]
   }
 
-  function isMlUnit(name) {
+  const UNIT_LABELS = {
+    drop: { zh: '滴', en: 'drop', enPlural: 'drops' },
+    ml: { zh: 'ml', en: 'ml', enPlural: 'ml' },
+    g: { zh: 'g', en: 'g', enPlural: 'g' },
+    capsule: { zh: '颗', en: 'capsule', enPlural: 'capsules' },
+  }
+
+  function getUnit(name) {
     const meta = oilsMeta.value[name]
-    return meta && meta.unit === 'ml'
+    return (meta && meta.unit) || 'drop'
+  }
+
+  function isDropUnit(name) {
+    return getUnit(name) === 'drop'
+  }
+
+  function isMlUnit(name) {
+    return getUnit(name) === 'ml'
   }
 
   function isPortionUnit(name) {
-    return isMlUnit(name)
+    return !isDropUnit(name)
   }
 
   function unitLabel(name, lang = 'zh') {
-    if (isMlUnit(name)) return 'ml'
-    if (name === '植物空胶囊') return lang === 'en' ? 'capsule' : '颗'
-    return lang === 'en' ? 'drop' : '滴'
+    const u = UNIT_LABELS[getUnit(name)] || UNIT_LABELS.drop
+    return lang === 'en' ? u.en : u.zh
   }
 
   function unitLabelPlural(name, count, lang = 'zh') {
-    if (isMlUnit(name)) return 'ml'
-    if (name === '植物空胶囊') return lang === 'en' ? (count === 1 ? 'capsule' : 'capsules') : '颗'
-    return lang === 'en' ? (count === 1 ? 'drop' : 'drops') : '滴'
+    const u = UNIT_LABELS[getUnit(name)] || UNIT_LABELS.drop
+    if (lang === 'en') return count === 1 ? u.en : u.enPlural
+    return u.zh
   }
 
   return {
@@ -127,6 +141,8 @@ export const useOilsStore = defineStore('oils', () => {
     loadOils,
     saveOil,
     deleteOil,
+    getUnit,
+    isDropUnit,
     isMlUnit,
     isPortionUnit,
     unitLabel,