Fix Umbrel setup completion and SV2 connection guidance

[keegreil]

Summary

• replace restart-sensitive setup auto-redirect polling with an explicit dashboard button
• expose the native SV2 Noise authority public key in the dashboard API
• display the authority key and AxeOS configuration guidance on setup and dashboard connection panels
• support package injection of the public authority key without exposing its private key

Validation

• 24 frontend tests pass and the production dashboard builds
• 239 backend tests pass
• live Bitaxe connected to Umbrel native SV2 after configuring the displayed authority key and accepted 30+ shares with zero rejects
• GridPool reports the Bitaxe as an active SV2 source at approximately 1 TH/s