GraphQL mode ignores manual Content-Type header override

When using a GraphQL request in Yaak, the Content-Type header appears to be set internally and cannot be overridden by manually adding a Content-Type header in the Headers tab.

Use case: I’m working against a GraphQL server that has introspection disabled. The server does not expose its schema and treats the payload as plain JSON. When I try to set a custom Content-Type (e.g. application/json with specific parameters, or any non-default value), the header I define is either ignored or the request is sent with the value Yaak decides internally.