feat: 套装成本扣除配件价值后再分摊

芳香调理-香薰机¥375，家庭医生-香薰机+木盒¥475，全精油-香薰机+竹木架¥575

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

frontend/src/composables/useKitCost.js

@@ -37,9 +37,9 @@ export function useKitCost() {
     }
     if (totalBottlePrice === 0) return {}
 
-    // Proportional allocation — kit accessories treated as freebies,
-    // so oil cost = min(kit price, sum of bottle prices)
-    const effectivePrice = Math.min(kit.price, totalBottlePrice)
+    // Proportional allocation — subtract accessory value, then cap at bottle price sum
+    const oilBudget = kit.price - (kit.accessoryValue || 0)
+    const effectivePrice = Math.min(oilBudget, totalBottlePrice)
     const perDrop = {}
     for (const name of resolved) {
       const meta = oils.oilsMeta[name]