[Desktop client] Briefly visible MFA screen when posture checks fail

[j-chmielewski]

When connecting to a location that requires MFA and posture checks, the UI briefly switches to the MFA screen before the /client-mfa/start response arrives. If posture validation fails, the app then redirects to the posture error screen, causing a visible “blinking MFA screen.”
This happens because the UI transitions to the MFA view optimistically, before knowing whether MFA can actually start.

Possible Solutions

1. Send the MFA start request on Connect click, then pass the successful response into the MFA screen. This avoids showing MFA unless it is valid, but may complicate the planned multi-MFA flow.
2. Keep the current navigation, but show a loader on the MFA screen until the MFA start response arrives.