FR: Switching tailnets without reauthentication (iOS, others?)

### What are you trying to do?

On two separate occasions where I’ve shared my tailnet with users, they’ve been confounded by the fact they had to log-out and then back in (this time selecting my tailnet) in order to switch between their tailnet and mine.

One of them already uses Tailscale and is quite “techy”, while the other is less experienced with Tailscale.

### How should we solve this?

Allow switching between Tailnet X and Tailnet Y without reauthenticating on iOS (and others?)

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

Tailnet sharing becomes a hassle for people who already use Tailscale, especially for the share recipient.

### Anything else?

_No response_