[New UI] Creating new wireguard device when network is full result in unclear error

[jakub-tldr]

Description

In this case, I have x.x.x.x/29 network which can contain 5 devices, creating 6th device result in unclear error, we should inform user more clearly about this.

Screen.Recording.2026-03-03.at.20.27.29.mov

[teon]

• We should vierify if there are free IPs before adding new wireguard device
• Display error toast: There are no free IP adresses in the network - please change the network settings.