fix: 编辑配方时正确识别容量和稀释比例

- 补充15ml/20ml的容量匹配
- 稀释比例取最近的可选值（3-20）
- 自定义时显示ml而非drops
- 无椰子油时默认单次模式

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

frontend/src/views/RecipeManager.vue

@@ -738,17 +738,33 @@ function calcDilutionFromIngs(ingredients) {
   const eoDrops = ings.filter(i => i.oil && i.oil !== '椰子油').reduce((s, i) => s + (i.drops || 0), 0)
   const cocoDrops = coco ? (coco.drops || 0) : 0
   const totalDrops = eoDrops + cocoDrops
+
+  // Set dilution ratio
   if (eoDrops > 0 && cocoDrops > 0) {
-    formDilution.value = Math.round(cocoDrops / eoDrops)
+    const ratio = Math.round(cocoDrops / eoDrops)
+    // Clamp to available options
+    const options = [3,4,5,6,7,8,9,10,12,15,20]
+    formDilution.value = options.reduce((prev, curr) => Math.abs(curr - ratio) < Math.abs(prev - ratio) ? curr : prev)
   }
+
+  // Set coconut drops for single mode
+  if (coco) {
+    formCocoRow.value.drops = cocoDrops
+  }
+
   // Guess volume
-  const DROPS_PER_ML = 18.6
-  const ml = totalDrops / DROPS_PER_ML
-  if (ml <= 1.5) formVolume.value = 'single'
-  else if (Math.abs(ml - 5) < 1.5) formVolume.value = '5'
-  else if (Math.abs(ml - 10) < 3) formVolume.value = '10'
-  else if (Math.abs(ml - 30) < 8) formVolume.value = '30'
-  else { formVolume.value = 'custom'; formCustomVolume.value = Math.round(totalDrops); formCustomUnit.value = 'drops' }
+  if (!coco || cocoDrops === 0) {
+    formVolume.value = 'single'
+  } else {
+    const ml = totalDrops / DROPS_PER_ML
+    if (ml <= 2) formVolume.value = 'single'
+    else if (Math.abs(ml - 5) < 1.5) formVolume.value = '5'
+    else if (Math.abs(ml - 10) < 2.5) formVolume.value = '10'
+    else if (Math.abs(ml - 15) < 2.5) formVolume.value = '15'
+    else if (Math.abs(ml - 20) < 3) formVolume.value = '20'
+    else if (Math.abs(ml - 30) < 6) formVolume.value = '30'
+    else { formVolume.value = 'custom'; formCustomVolume.value = Math.round(ml) }
+  }
 }
 
 function editRecipe(recipe) {