fix(backend): 修复 golangci-lint 报告的格式和代码规范问题

- gofmt: 修复 account_handler.go, models.go, gemini_messages_compat_service.go 的格式
- staticcheck ST1005: 将 error strings 改为小写开头

backend/internal/pkg/gemini/models.go

@@ -40,4 +40,3 @@ func FallbackModel(model string) Model {
 	}
 	return Model{Name: "models/" + model, SupportedGenerationMethods: methods}
 }
-