fix: 配方编辑器禁止重复添加同一精油

选择已存在的精油时提示"请直接修改滴数"。

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

frontend/src/views/RecipeManager.vue

@@ -864,6 +864,13 @@ function filteredOilNames(search) {
 }
 
 function selectOil(ing, name) {
+  // Check for duplicate oil in current recipe
+  const existing = formIngredients.value.find(i => i !== ing && i.oil === name)
+  if (existing) {
+    ui.showToast(`已有「${name}」，请直接修改滴数`)
+    ing._open = false
+    return
+  }
   ing.oil = name
   ing._search = name
   ing._open = false