mark messages as read when selecting a thread

Updates local flags and enqueues \\Seen flag action for IMAP sync.

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

Apps/MagnumOpus/ViewModels/MailViewModel.swift

@@ -187,7 +187,7 @@ final class MailViewModel {
 	func selectThread(_ thread: ThreadSummary) {
 		selectedThread = thread
 		messageObservation?.cancel()
-		guard let store else { return }
+		guard let store, let accountConfig else { return }
 		messageObservation = Task {
 			do {
 				for try await msgs in store.observeMessages(threadId: thread.id) {
@@ -199,6 +199,24 @@ final class MailViewModel {
 				}
 			}
 		}
+
+		// Mark all unread messages in the thread as read
+		if thread.unreadCount > 0 {
+			do {
+				let messages = try store.messagesForThread(threadId: thread.id)
+				for message in messages where !message.isRead {
+					try store.updateFlags(messageId: message.id, isRead: true, isFlagged: message.isFlagged)
+					let action = PendingAction(
+						accountId: accountConfig.id,
+						actionType: .setFlags,
+						payload: .setFlags(uid: message.uid, mailbox: mailboxName(for: message.mailboxId) ?? "INBOX", add: ["\\Seen"], remove: [])
+					)
+					Task { try await actionQueue?.enqueue(action) }
+				}
+			} catch {
+				errorMessage = error.localizedDescription
+			}
+		}
 	}
 
 	private var isSyncing = false