FR: Provide a way to copy the Tailnet name to the clipboard in client

### What are you trying to do?

When using the Tailscale client it is possible to get another device's IP address added to the system clipboard by selecting the device in the client.

It would be equally useful to have the current tailnet's FQDN (tailXXXX.ts.net) pasted unto the clipboard from an option in the client menus.

### How should we solve this?

For example, in the MacOS client clicking Tailscale menubar icon, a list of menu options is presented. The second option shows the current device name and the Tailnet's FQDN. Clicking this menu item (rather than one of the menu items presented when it is pointed at) could paste the Tailnet's FQDN into the clipboard.

This would be consistent with clicking on a device on the devices menu pasting the device's IP address.

### What is the impact of not solving this?

This would avoid lots and lots of typing of my Tailsnet's FQDN :-)

### Anything else?

_No response_