refactor: 简化 trial 赠送配置，删除多余的白名单启用开关

**简化方案**:
- 删除 `EnableTrialEmailWhitelist` 配置字段（多余）
- 保留 `TrialEmailDomainWhitelist`（逗号分隔的白名单域名）
- 赠送规则：白名单**非空** 且 **邮箱在列表** 才赠送

**新的赠送逻辑**:
```yaml
TrialEmailDomainWhitelist: "qq.com,163.com,gmail.com"
```
- 为空 → ❌ 不赠送（关闭）
- 非空 → ✅ 赠送给白名单域名的邮箱

**更新的地方**:
1. internal/config/config.go - 删除 EnableTrialEmailWhitelist 字段
2. userRegisterLogic.go - 简化赠送逻辑
3. emailLoginLogic.go - 简化赠送逻辑
4. bindEmailWithVerificationLogic.go - 简化赠送逻辑
5. oAuthLoginGetTokenLogic.go - 简化赠送逻辑

**配置升级说明**:
旧配置:
```yaml
EnableTrial: true
EnableTrialEmailWhitelist: true
TrialEmailDomainWhitelist: "qq.com,163.com"
```

新配置（只保留两项）:
```yaml
EnableTrial: true
TrialEmailDomainWhitelist: "qq.com,163.com"
```

关闭赠送:
```yaml
EnableTrial: true
TrialEmailDomainWhitelist: ""
```

Co-Authored-By: claude-flow <ruv@ruv.net>

internal/config/config.go

@@ -87,12 +87,11 @@ type RegisterConfig struct {
 	TrialSubscribe          int64  `yaml:"TrialSubscribe" default:"0"`
 	TrialTime               int64  `yaml:"TrialTime" default:"0"`
 	TrialTimeUnit           string `yaml:"TrialTimeUnit" default:""`
+	TrialEmailDomainWhitelist string `yaml:"TrialEmailDomainWhitelist" default:""`
 	IpRegisterLimit         int64  `yaml:"IpRegisterLimit" default:"0"`
 	IpRegisterLimitDuration int64  `yaml:"IpRegisterLimitDuration" default:"0"`
 	EnableIpRegisterLimit   bool   `yaml:"EnableIpRegisterLimit" default:"false"`
 	DeviceLimit               int64  `yaml:"DeviceLimit" default:"2"`
-	EnableTrialEmailWhitelist bool   `yaml:"EnableTrialEmailWhitelist" default:"false"`
-	TrialEmailDomainWhitelist string `yaml:"TrialEmailDomainWhitelist" default:""`
 }
 
 type EmailConfig struct {