BUG: Memory leak in IMAP IDLE connection management

[andrinoff]

Describe the bug: IDLE watchers accumulate over time. When switching accounts or reconnecting, old goroutines may not terminate properly. No cleanup verification in StopAllAndWait().

To reproduce:

1. Connect to IMAP account with IDLE enabled
2. Switch accounts multiple times
3. Monitor goroutines - count increases
4. Old IDLE watchers remain active

Expected behavior: All goroutines should terminate when StopAll() called. Verify with goroutine count decrease.

Additional context:

• File: fetcher/idle.go
• Related to race condition in BUG: Potential race condition in IDLE watcher Stop() #617 but more complex
• Requires goroutine leak detection
• May need runtime.NumGoroutine() instrumentation
• Severity: Medium-High - resource leak over time
• Complexity: High - concurrency debugging