fix(ios): avoid redundant message refreshes

Skip replacing the message dictionary when polling returns the same message list, avoiding unnecessary SwiftUI refreshes while keeping the Xcode 15-safe assignment path.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: hyochan

clients/ios/DeeplineIOS/Sources/DeeplineAppModel.swift

@@ -474,6 +474,7 @@ final class DeeplineAppModel: ObservableObject {
     }
 
     private func replaceMessages(_ newMessages: [DeeplineMessage], for conversationId: String) {
+        guard messages[conversationId] != newMessages else { return }
         var updatedMessages = messages
         updatedMessages[conversationId] = newMessages
         messages = updatedMessages