fix code review issues: deferral date format, storeFlags SELECT, event loop leaks, GTD selection tracking

- fix deferral resurfacing using VTODOParser.formatDateOnly instead of ISO8601 (date format mismatch)
- add SELECT mailbox before UID STORE in storeFlags (IMAP protocol requirement)
- pass credentials to SyncCoordinator so IDLE monitoring activates
- add selectedItem tracking to MailViewModel, wire List selection in GTD views
- fix startPeriodicSync to sleep-first, preventing duplicate sync on launch
- add deinit cleanup for EventLoopGroup in IMAPConnection, SMTPConnection
- use separate IMAP client for attachment downloads, avoid shared connection interference
- remove [weak self] from IMAPIdleClient actor Task to prevent orphaned connections
- fix isGTDPerspective to check selectedMailbox instead of items.isEmpty
- fix fetchBody to use complete RFC822 fetch instead of BODY[TEXT]
- reuse single IMAP connection per ActionQueue.flush() batch
- add requiresIMAP to ActionPayload for connection batching
- load task categories from label store instead of hardcoded empty array
- suppress NIOSSLHandler Sendable warnings via Package.swift unsafeFlags
- fix unused variable warnings across codebase

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

Packages/MagnumOpusCore/Sources/IMAPClient/IMAPConnection.swift

@@ -1,7 +1,7 @@
 import NIO
 import NIOIMAPCore
-@preconcurrency import NIOIMAP
-@preconcurrency import NIOSSL
+import NIOIMAP
+import NIOSSL
 
 actor IMAPConnection {
 	private let host: String
@@ -17,16 +17,20 @@ actor IMAPConnection {
 		self.responseHandler = IMAPResponseHandler()
 	}
 
+	deinit {
+		try? group.syncShutdownGracefully()
+	}
+
 	func connect() async throws {
 		let sslContext = try NIOSSLContext(configuration: TLSConfiguration.makeClientConfiguration())
 		let handler = responseHandler
 		let hostname = host
 
+		nonisolated(unsafe) let sslHandler = try NIOSSLClientHandler(context: sslContext, serverHostname: hostname)
 		let bootstrap = ClientBootstrap(group: group)
 			.channelOption(ChannelOptions.socketOption(.so_reuseaddr), value: 1)
 			.channelInitializer { channel in
-				let sslHandler = try! NIOSSLClientHandler(context: sslContext, serverHostname: hostname)
-				return channel.pipeline.addHandlers([
+				channel.pipeline.addHandlers([
 					sslHandler,
 					IMAPClientHandler(),
 					handler,