fix: 注册 PUT /api/manager/settings 使设置保存生效

server.go NewServeMux 按方法分发到 handleSettingsPut（原仅注册 GET，PUT 静默返回旧值）

新增 TestSettingsPutRoundTrip 回归测试；gofmt 清理历史格式问题

internal/httpapi/handlers.go

@@ -257,8 +257,6 @@ func (h *Handler) handleProfile(w http.ResponseWriter, r *http.Request) {
 
 // ---- helpers ----
 
-
-
 // tailFile returns the last maxBytes bytes of a file.
 func tailFile(path string, maxBytes int64) (string, error) {
 	info, err := os.Stat(path)
@@ -280,4 +278,3 @@ func tailFile(path string, maxBytes int64) (string, error) {
 	}
 	return string(buf), nil
 }
-