IMAP: retry with exponential backoff on connection failures

[wesm]

Context

The IMAP client in #193 makes a single reconnect attempt on network errors. For large mailboxes synced over unreliable connections (or servers that throttle aggressive clients), this can cause sync failures that require manual re-runs.

Proposal

Add configurable retry with exponential backoff for transient network errors:

• 3 attempts with backoff (e.g. 5s, 15s, 45s)
• Only retry on connection-level errors (EOF, reset, timeout) — not auth or protocol errors
• Log each retry attempt at warn level

The retry logic could live in withConn() or a wrapper around the connect/reconnect path.

References

• Yahoo Mail PRD: Design for Yahoo Mail integration #93
• IMAP support: Add IMAP account support via go-imap/v2 #193