Backend: cache status in RwLock, refresh every 15s via background task.
Eliminates per-request TCP race to ElectrumX that caused volatile errors.
Fix error classification so "Failed to read" is transient, not hard error.
Frontend: keep last-known-good data across failed polls, persist Tor
onion once discovered, adaptive polling (5s active / 30s synced).
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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