fix: 切换到单次模式时自动重置椰子油滴数（>30滴→10滴）

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

frontend/src/views/RecipeManager.vue

@@ -456,10 +456,14 @@ const formDilution = ref(6)
 
 const formCocoRow = ref(null)
 
-watch(() => formVolume.value, (vol) => {
+watch(() => formVolume.value, (vol, oldVol) => {
   if (vol && !formCocoRow.value && parsedCurrentIndex.value < 0) {
     formCocoRow.value = { oil: '椰子油', drops: vol === 'single' ? 10 : 0, _search: '椰子油', _open: false }
   }
+  // Switching to single: reset coconut drops if they're too large (leftover from fill mode)
+  if (vol === 'single' && formCocoRow.value && formCocoRow.value.drops > 30) {
+    formCocoRow.value.drops = 10
+  }
 })
 
 // EO ingredients (everything except coconut)