fix: 套装折扣改为统一折扣率，配件和精油共享同一折扣

折扣率 = 套装价 / (油瓶总价 + 配件零售价)
芳香调理32.4%, 家庭医生32.8%, 居家呵护36.1%

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

frontend/src/composables/useKitCost.js

@@ -37,15 +37,17 @@ export function useKitCost() {
     }
     if (totalBottlePrice === 0) return {}
 
-    // Proportional allocation — subtract accessory value, then cap at bottle price sum
-    const oilBudget = kit.price - (kit.accessoryValue || 0)
-    const effectivePrice = Math.min(oilBudget, totalBottlePrice)
+    // Uniform discount: kit price covers oils + accessories at the same discount rate
+    // discount_rate = kit_price / (oil_total + accessory_value)
+    // each oil's kit cost = bottle_price × discount_rate
+    const totalValue = totalBottlePrice + (kit.accessoryValue || 0)
+    const discountRate = Math.min(kit.price / totalValue, 1) // cap at 1 (no markup)
     const perDrop = {}
     for (const name of resolved) {
       const meta = oils.oilsMeta[name]
       const count = bc[name] || 1
       const bp = oilBottlePrices[name]
-      const kitCostForOil = (bp / totalBottlePrice) * effectivePrice
+      const kitCostForOil = bp * discountRate
       const totalDrops = meta ? meta.dropCount * count : 1
       perDrop[name] = totalDrops > 0 ? kitCostForOil / totalDrops : 0
     }