feat(send,receive): the ambient scene, as a layout item this time

Send had a large empty region under the form doing nothing, and Receive had
nothing at all. Both get the scene back — but as real layout items, not as the
overlay that painted over the forms last time.

Send: the form only ever needed the top of the tab, so the QFormLayout is now
wrapped in a vbox and the scene takes the rest. The widget's vertical size
policy was Maximum, which made it hug the form; it owns the whole tab now. The
form itself is untouched, and nothing is drawn on top of the address or amount
fields.

Receive: a short banner under the address table.

Built and smoke-tested from the AppImage lane.

src/SendWidget.cpp

@@ -11,6 +11,7 @@
 #include "Icons.h"
 #include "libwalletqt/Wallet.h"
 #include "libwalletqt/WalletManager.h"
+#include "widgets/SceneWidget.h"
 
 #if defined(WITH_SCANNER)
 #include "wizard/offline_tx_signing/OfflineTxSigningWizard.h"
@@ -25,6 +26,14 @@ SendWidget::SendWidget(Wallet *wallet, QWidget *parent)
 {
     ui->setupUi(this);
 
+    // wowlet: the form only needs the top of the tab — give the rest of it to
+    // the ambient scene. A layout item under the form, never an overlay: the
+    // send form itself stays untouched.
+    auto *scene = new SceneWidget(this);
+    scene->setMinimumHeight(160);
+    scene->setSizePolicy(QSizePolicy::Expanding, QSizePolicy::Expanding);
+    ui->verticalLayoutOuter->addWidget(scene, 1);
+
     QString amount_rx = R"(^\d{0,8}[\.,]\d{0,12}|(all)$)";
     QRegularExpression rx;
     rx.setPattern(amount_rx);