refactor: 移除旧版数据库配置的简易模式实现

移除与 PR #66 冲突的旧版简易模式实现（commit 7d4b7de）。
新版简易模式通过 run_mode 配置文件/环境变量控制，无需数据库设置。

后端变更：
- 移除 SettingKeySimpleMode 常量
- 移除 SystemSettings/PublicSettings 中的 SimpleMode 字段
- 移除 setting_handler 中的简易模式切换逻辑
- 移除 userService 依赖（不再需要自动设置管理员并发数）

前端变更：
- 移除 appStore.simpleMode 状态
- 移除设置页面的"使用模式"设置区块
- 移除 GroupsView 中的简易模式相关逻辑
- 移除相关国际化文案

backend/internal/handler/admin/setting_handler.go

@@ -12,15 +12,13 @@ import (
 type SettingHandler struct {
 	settingService *service.SettingService
 	emailService   *service.EmailService
-	userService    *service.UserService
 }
 
 // NewSettingHandler 创建系统设置处理器
-func NewSettingHandler(settingService *service.SettingService, emailService *service.EmailService, userService *service.UserService) *SettingHandler {
+func NewSettingHandler(settingService *service.SettingService, emailService *service.EmailService) *SettingHandler {
 	return &SettingHandler{
 		settingService: settingService,
 		emailService:   emailService,
-		userService:    userService,
 	}
 }
 
@@ -54,7 +52,6 @@ func (h *SettingHandler) GetSettings(c *gin.Context) {
 		DocUrl:              settings.DocUrl,
 		DefaultConcurrency:  settings.DefaultConcurrency,
 		DefaultBalance:      settings.DefaultBalance,
-		SimpleMode:          settings.SimpleMode,
 	})
 }
 
@@ -89,9 +86,6 @@ type UpdateSettingsRequest struct {
 	// 默认配置
 	DefaultConcurrency int     `json:"default_concurrency"`
 	DefaultBalance     float64 `json:"default_balance"`
-
-	// 使用模式
-	SimpleMode bool `json:"simple_mode"`
 }
 
 // UpdateSettings 更新系统设置
@@ -114,14 +108,8 @@ func (h *SettingHandler) UpdateSettings(c *gin.Context) {
 		req.SmtpPort = 587
 	}
 
-	// 简单模式下自动关闭开放注册
-	registrationEnabled := req.RegistrationEnabled
-	if req.SimpleMode {
-		registrationEnabled = false
-	}
-
 	settings := &service.SystemSettings{
-		RegistrationEnabled: registrationEnabled,
+		RegistrationEnabled: req.RegistrationEnabled,
 		EmailVerifyEnabled:  req.EmailVerifyEnabled,
 		SmtpHost:            req.SmtpHost,
 		SmtpPort:            req.SmtpPort,
@@ -141,7 +129,6 @@ func (h *SettingHandler) UpdateSettings(c *gin.Context) {
 		DocUrl:              req.DocUrl,
 		DefaultConcurrency:  req.DefaultConcurrency,
 		DefaultBalance:      req.DefaultBalance,
-		SimpleMode:          req.SimpleMode,
 	}
 
 	if err := h.settingService.UpdateSettings(c.Request.Context(), settings); err != nil {
@@ -149,14 +136,6 @@ func (h *SettingHandler) UpdateSettings(c *gin.Context) {
 		return
 	}
 
-	// 如果切换到简单模式，自动将管理员并发数设为 99999
-	if req.SimpleMode && h.userService != nil {
-		admin, err := h.userService.GetFirstAdmin(c.Request.Context())
-		if err == nil && admin != nil {
-			_ = h.userService.UpdateConcurrency(c.Request.Context(), admin.ID, 99999)
-		}
-	}
-
 	// 重新获取设置返回
 	updatedSettings, err := h.settingService.GetAllSettings(c.Request.Context())
 	if err != nil {
@@ -185,7 +164,6 @@ func (h *SettingHandler) UpdateSettings(c *gin.Context) {
 		DocUrl:              updatedSettings.DocUrl,
 		DefaultConcurrency:  updatedSettings.DefaultConcurrency,
 		DefaultBalance:      updatedSettings.DefaultBalance,
-		SimpleMode:          updatedSettings.SimpleMode,
 	})
 }