统一角色体系为admin/agent/user，修复代理查询角色错误，添加语言切换和明暗切换

backend/internal/middleware/middleware.go

@@ -114,7 +114,7 @@ func AdminAuth() gin.HandlerFunc {
 	}
 }
 
-// DeveloperAuth 开发者权限中间件
+// DeveloperAuth 开发者权限中间件（管理员即开发者）
 func DeveloperAuth() gin.HandlerFunc {
 	return func(c *gin.Context) {
 		role, exists := c.Get("role")
@@ -124,8 +124,8 @@ func DeveloperAuth() gin.HandlerFunc {
 			return
 		}
 
-		if role != "developer" && role != "admin" {
-			response.Error(c, http.StatusForbidden, "Developer access required")
+		if role != "admin" {
+			response.Error(c, http.StatusForbidden, "Admin access required")
 			c.Abort()
 			return
 		}