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/database/database.go

@@ -308,7 +308,7 @@ func initData() {
 			Username: "dev",
 			Email:    &devEmail,
 			Password: string(hashedPassword),
-			Role:     "developer",
+			Role:     "admin",
 			Status:   "active",
 		}
 		DB.Create(&devUser)
@@ -324,7 +324,7 @@ func initData() {
 			Username: "agent",
 			Email:    &agentEmail,
 			Password: string(hashedPassword),
-			Role:     "developer",
+			Role:     "agent",
 			Status:   "active",
 		}
 		DB.Create(&agentUser)
@@ -678,7 +678,7 @@ func initTicketData() {
 				Username: "dev",
 				Email:    &devEmail,
 				Password: string(hashedPassword),
-				Role:     "developer",
+				Role:     "admin",
 				Status:   "active",
 			}
 			DB.Create(&devUser)