feat: 支付手续费功能

- 支持提供商级别和渠道级别手续费率配置（FEE_RATE_PROVIDER_* / FEE_RATE_*）
- 用户多付手续费，到账金额不变（充值 ¥100 + 1.6% = 实付 ¥101.60）
- 前端显示手续费明细和实付金额
- 退款时按实付金额退款，余额扣减到账金额

src/__tests__/lib/payment/registry.test.ts

@@ -12,10 +12,12 @@ import type {
 
 class MockProvider implements PaymentProvider {
   readonly name: string;
+  readonly providerKey: string;
   readonly supportedTypes: PaymentType[];
 
   constructor(name: string, types: PaymentType[]) {
     this.name = name;
+    this.providerKey = name;
     this.supportedTypes = types;
   }