fix offline: remove deadlocking syncShutdownGracefully from deinit, limit flag sync range

- remove deinit from IMAPConnection, SMTPConnection — syncShutdownGracefully
  blocks and deadlocks if called on NIO event loop thread
- limit reconcileFlags to last 500 UIDs to avoid overwhelming the connection

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

Packages/MagnumOpusCore/Sources/IMAPClient/IMAPConnection.swift

@@ -17,10 +17,6 @@ actor IMAPConnection {
 		self.responseHandler = IMAPResponseHandler()
 	}
 
-	deinit {
-		try? group.syncShutdownGracefully()
-	}
-
 	func connect() async throws {
 		let sslContext = try NIOSSLContext(configuration: TLSConfiguration.makeClientConfiguration())
 		let handler = responseHandler

Packages/MagnumOpusCore/Sources/SMTPClient/SMTPConnection.swift

@@ -19,10 +19,6 @@ actor SMTPConnection {
 		self.responseHandler = SMTPResponseHandler()
 	}
 
-	deinit {
-		try? group.syncShutdownGracefully()
-	}
-
 	func connect() async throws -> SMTPResponse {
 		let handler = responseHandler
 		let hostname = host

Packages/MagnumOpusCore/Sources/SyncEngine/SyncCoordinator.swift

@@ -171,9 +171,10 @@ public final class SyncCoordinator {
 			emit(.newMessages(count: envelopes.count, mailbox: remoteMailbox.name))
 		}
 
-		// Reconcile flags for existing messages (read/unread state from other devices)
+		// Reconcile flags for recent existing messages (read/unread state from other devices)
 		if lastUid > 0 {
-			await reconcileFlags(mailboxId: mailboxId, uidRange: 1...lastUid)
+			let flagSyncStart = max(1, lastUid - 499)
+			await reconcileFlags(mailboxId: mailboxId, uidRange: flagSyncStart...lastUid)
 		}
 
 		await prefetchBodies(mailboxId: mailboxId)