In matters deemed prohibited, we do not rely on intentions or speculation; rather, the prohibited should be based on certainty. Otherwise, it is considered permissible as the original status of things is permissibility.

Manufacturing a television is not forbidden, even though what is broadcast on it may be prohibited. This is because it is just a tool that can be used for both good and evil. The goodness or sin lies with the user, not the manufacturer. Similarly, a knife can be used for killing or lawful slaughter of sacrificial animals according to Islamic law, and the distinction lies in how it is used.

You are responsible for designing something that is originally permissible. If some people misuse it for impermissible purposes, the responsibility lies with them. Allah may replace them with others who use it for lawful purposes. It is not required in your work to specify the intended use for your designs.

The restaurant you will design is, in its essence, a permissible structure, similar to a hotel, prison, and others. If the hotel is used for immoral activities or the prison is used to unlawfully restrict the freedom of innocent people, the responsibility lies with the users, not the designer.

The prohibition only applies if you, as the designer, undertake the design with the explicit intention of facilitating the prohibited act. In that case, your intention would be linked to the prohibited action. Otherwise, there is no prohibition.