fix(BRE2-815): Halt on netbird err

[drewmalin]

Currently we treat this as a warning and proceed with the (doomed to fail) SSH installation.

[Step 3/3] Registering with Brev...
  Registration complete.
registration failed: setup command failed: setup command failed: exit status 1

[drewmalin]

This is solved by the backoff during ssh

[Copilot]

Pull request overview

This PR changes the behavior of the runSetup function in the registration flow so that a missing or failed netbird tunnel setup now halts registration with a proper error, rather than silently warning and proceeding to the (doomed-to-fail) SSH installation step. The terminal.Terminal parameter is also removed from runSetup since it's no longer used for printing warnings.

Changes:

• runSetup is refactored to return an error instead of printing warnings via the terminal.
• runRegister is updated to propagate the runSetup error and abort registration.
• The time.Sleep(2 * time.Second) that settled network routes after a successful netbird setup is silently dropped as a side effect of the refactor.

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