FR: Hide "My devices" unless you are logged in

### What are you trying to do?

I would prefer if the "my devices" option was not available if the PC is not currently logged into the tailscale profile.

Currently a device can see all devices on a tailnet if they are not logged in.

### How should we solve this?

Remove "my devices" unless the user is logged in.

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

Potential privacy issues.

### Anything else?

_No response_

The issue names no files, tests, or entry points. Start by locating the “My devices” UI and the logged-in state, then verify that the option is absent when logged out and present when logged in.

Written by the indexing model from the issue text.