Fix Qt widget embedding: pages rendered as floating windows instead of tabs

Move centralWidget assignment before page creation to fix use of
uninitialized pointer. Use Qt::Widget flags when pages have a parent
(embedded in QStackedWidget) and pass centralWidget as parent for all
lazily-created pages (messagePage, signMessagePage, verifyMessagePage).
Also fix TransactionView which unconditionally set FramelessWindowHint.

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

src/qt/sendcoinsdialog.cpp

@@ -28,7 +28,10 @@ SendCoinsDialog::SendCoinsDialog(QWidget *parent) :
     model(0)
 {
     ui->setupUi(this);
-    setWindowFlags(Qt::CustomizeWindowHint | Qt::FramelessWindowHint | Qt::Window);
+    if (parent)
+        setWindowFlags(Qt::Widget);
+    else
+        setWindowFlags(Qt::CustomizeWindowHint | Qt::FramelessWindowHint | Qt::Window);
 
 #ifdef Q_OS_MAC // Icons on push buttons are very uncommon on Mac
     ui->addButton->setIcon(QIcon());