fix sync: INSERT OR IGNORE for duplicate messages, fix leaked IMAP continuations

- insertMessages uses INSERT OR IGNORE on (mailboxId, uid) conflict instead of
  crashing on UNIQUE constraint when messages are re-fetched after restart
- IMAPResponseHandler.sendCommand resumes any leaked previous continuation before
  registering a new one, preventing DuplicateCommandTag errors
- add channelInactive handler to resume pending continuations on connection drop
- add error type to sync failure log for better diagnostics

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Apps/MagnumOpus/ViewModels/MailViewModel.swift

@@ -207,8 +207,10 @@ final class MailViewModel {
 			try await coordinator.syncNow()
 			syncState = coordinator.syncState
 		} catch {
-			errorMessage = error.localizedDescription
-			syncState = .error(error.localizedDescription)
+			let desc = "\(type(of: error)): \(error.localizedDescription)"
+			print("[MailViewModel] syncNow failed: \(desc)")
+			errorMessage = desc
+			syncState = .error(desc)
 		}
 	}