Advertise Tailscale Serve HTTPS URL in headless pairing output

[johnlombardo-dev]

Hi. I saw the note that the project is not currently accepting PRs, so please treat this primarily as a feature request. I have a small branch implementing this and am happy for you to use, adapt, or ignore the contribution as useful.

When running t3 serve --tailscale-serve, the server configures Tailscale Serve, but the startup/pairing output can still advertise the local HTTP URL. For remote/headless use this means the user has to manually translate the URL to the MagicDNS HTTPS address (like a caveman).

Requested behavior:

• After Tailscale Serve is configured, advertise the https://<machine>.<tailnet>.ts.net/ URL in startup and pairing output.
• Allow an explicit host override for environments where MagicDNS discovery is unavailable or undesirable.
• Fall back to the existing HTTP URL if Tailscale status lookup fails or Serve setup did not complete.

The change is low impact: it only affects advertised startup URLs when --tailscale-serve is enabled, and keeps the current behavior as fallback.

[johnlombardo-dev]

I opened a small implementation reference here: #2835.