重构: 手续费改为百分比/固定二选一，清理旧 Skin/Order/ShipLog 演示链路

手续费:
- 新增 fee_type 字段(rate/fixed)，百分比与固定金额二选一，不再叠加
- calculateServiceFee 按 fee_type 分支计算
- 商户创建/更新校验 fee_type，订单落库快照 fee_type
- 前端表单改为下拉选择手续费类型，动态显示对应输入框
- 测试拆为 TestFeeRate + TestFeeFixed

清理旧链路:
- 删除旧 Skin/Order/ShipLog 模型及 OrderService/SkinService
- Dashboard 迁移到 FulfillmentService
- 上游 /api/open/v1 改为基于 FulfillmentOrder 实现，接口契约不变
- 推送留痕改用 AuditLog，不再建 ShipLog 表
- 删除前端 Skins/Orders/ShipLogs/Distributors 页面及路由、菜单、API、类型
- 新增迁移 003: 添加 fee_type 列并 DROP 旧表

frontend/src/layouts/MainLayout.tsx

@@ -11,14 +11,10 @@ import {
 } from 'antd'
 import {
   DashboardOutlined,
-  SkinOutlined,
-  ShoppingOutlined,
-  TeamOutlined,
   UserOutlined,
   LogoutOutlined,
   MenuFoldOutlined,
   MenuUnfoldOutlined,
-  SendOutlined,
   ApiOutlined,
   ShopOutlined,
 } from '@ant-design/icons'
@@ -39,15 +35,11 @@ export default function MainLayout() {
   const menuItems: MenuProps['items'] = useMemo(() => {
     const items: MenuProps['items'] = [
       { key: '/', icon: <DashboardOutlined />, label: '数据概览' },
-      { key: '/skins', icon: <SkinOutlined />, label: '皮肤商品' },
-      { key: '/orders', icon: <ShoppingOutlined />, label: '订单管理' },
       { key: '/merchant-center', icon: <ShopOutlined />, label: '商户中心' },
     ]
     if (isAdmin) {
       items.push(
-        { key: '/distributors', icon: <TeamOutlined />, label: '分销商' },
         { key: '/platform-merchants', icon: <ShopOutlined />, label: '商户管理' },
-        { key: '/ship-logs', icon: <SendOutlined />, label: '发货记录' },
         { key: '/open-api', icon: <ApiOutlined />, label: '开放接口' },
       )
     }
@@ -81,7 +73,7 @@ export default function MainLayout() {
             letterSpacing: 1,
           }}
         >
-          {collapsed ? '皮肤' : '皮肤分销系统'}
+          {collapsed ? '供货' : '皮肤供货平台'}
         </div>
         <Menu
           theme="dark"
@@ -112,7 +104,7 @@ export default function MainLayout() {
               <Avatar size="small" icon={<UserOutlined />} />
               <Typography.Text>
                 {user?.nickname || user?.username}
-                {isAdmin ? '（管理员）' : '（分销商）'}
+                {isAdmin ? '（平台管理员）' : '（商户账号）'}
               </Typography.Text>
             </Space>
           </Dropdown>