feat: 配方显示容量和可做次数；套装成本不超过单买价（配件视为赠品）

- 每个配方名后显示容量（单次/5ml/10ml等）
- 新增可做次数列（按最先用完的精油计算）
- 套装成本分摊使用 min(套装价, 油瓶总价)，避免含配件的套装算出比单买贵

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

frontend/src/composables/useKitCost.js

@@ -35,12 +35,14 @@ export function useKitCost() {
     }
     if (totalBottlePrice === 0) return {}
 
-    // Proportional allocation
+    // Proportional allocation — kit accessories treated as freebies,
+    // so oil cost = min(kit price, sum of bottle prices)
+    const effectivePrice = Math.min(kit.price, totalBottlePrice)
     const perDrop = {}
     for (const name of resolved) {
       const meta = oils.oilsMeta[name]
       const bp = oilBottlePrices[name]
-      const kitCostForOil = (bp / totalBottlePrice) * kit.price
+      const kitCostForOil = (bp / totalBottlePrice) * effectivePrice
       const drops = meta ? meta.dropCount : 1
       perDrop[name] = drops > 0 ? kitCostForOil / drops : 0
     }
@@ -124,6 +126,7 @@ export function useKitCost() {
         id,
         name: recipe.name,
         tags: recipe.tags,
+        volume: recipe.volume,
         ingredients: recipe.ingredients,
         originalCost: recipe.originalCost,
         costs,