Defer heavy UI model loading for faster startup

- Address table and transaction table now load asynchronously via
  QTimer::singleShot after the event loop starts instead of blocking
  the constructor
- MessageModel (secure messaging) lazy-loaded 1s after wallet init
  or on-demand when navigating to message page
- Splash screen stays visible until window.show() completes
- Guard updateConfirmations() against empty transaction table

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/qt/trianglesgui.cpp

@@ -95,6 +95,7 @@ TrianglesGUI::TrianglesGUI(bool fIsTestnet, QWidget *parent):
     ui(new Ui::MainWindow),
     clientModel(0),
     walletModel(0),
+    messageModel(0),
     encryptWalletAction(0),
     changePassphraseAction(0),
     unlockWalletAction(0),
@@ -594,6 +595,9 @@ void TrianglesGUI::setWalletModel(WalletModel *walletModel)
 
         // Ask for passphrase if needed
         connect(walletModel, SIGNAL(requireUnlock()), this, SLOT(unlockWallet()));
+
+        // Load secure messaging after the main window can render.
+        QTimer::singleShot(1000, this, SLOT(ensureMessageModel()));
     }
 }
 
@@ -614,6 +618,14 @@ void TrianglesGUI::setMessageModel(MessageModel *messageModel)
     }
 }
 
+void TrianglesGUI::ensureMessageModel()
+{
+    if(messageModel || !walletModel)
+        return;
+
+    setMessageModel(new MessageModel(pwalletMain, walletModel, this));
+}
+
 void TrianglesGUI::createTrayIcon()
 {
 #ifndef Q_OS_MAC
@@ -1025,6 +1037,8 @@ void TrianglesGUI::gotoSendCoinsPage()
 
 void TrianglesGUI::gotoMessagePage()
 {
+    ensureMessageModel();
+
     messageAction->setChecked(true);
     centralWidget->setCurrentWidget(messagePage);