FEAT: Implement IMAP IDLE timeout detection per RFC 2177

[andrinoff]

Is your feature request related to a problem?

IDLE connections don't detect server-side timeouts. RFC 2177 allows servers to drop IDLE after 29 minutes. Current code (fetcher/idle.go) has exponential backoff but no proactive timeout detection or heartbeat.

Describe the solution you'd like

• Add configurable IDLE timeout (default 25 min per RFC recommendation)
• Send DONE + re-IDLE before server timeout
• Detect network interruptions via TCP keepalive
• Implement fallback to polling when IDLE is unsupported or unreliable
• Add per-server IDLE capability detection

[floatpanebot]

This has had no activity for 30 days. It will be closed in 14 days unless updated. Comment or remove the stale label to keep it open.

[floatpanebot]

Closing due to inactivity. Reopen if still relevant.