qt: Choose monospaced font in C++ code rather in *.ui file

Setting the "Monospace" font family in a `*.ui` file does not work on
macOS, at least on Big Sur with Qt 5.15 (neither via the "font" property
nor via the "styleSheet" property). Qt chooses the ".AppleSystemUIFont"
instead of ".AppleSystemUIFontMonospaced".

This change makes macOS choose the correct monospaced font.

src/qt/overviewpage.cpp

@@ -257,6 +257,8 @@ void OverviewPage::setClientModel(ClientModel *model)
         // Show warning, for example if this is a prerelease version
         connect(model, &ClientModel::alertsChanged, this, &OverviewPage::updateAlerts);
         updateAlerts(model->getStatusBarWarnings());
+
+        setMonospacedFont(false);
     }
 }
 
@@ -321,3 +323,17 @@ void OverviewPage::showOutOfSyncWarning(bool fShow)
     ui->labelWalletStatus->setVisible(fShow);
     ui->labelTransactionsStatus->setVisible(fShow);
 }
+
+void OverviewPage::setMonospacedFont(bool use_embedded_font)
+{
+    QFont f = GUIUtil::fixedPitchFont(use_embedded_font);
+    f.setWeight(QFont::Bold);
+    ui->labelBalance->setFont(f);
+    ui->labelUnconfirmed->setFont(f);
+    ui->labelImmature->setFont(f);
+    ui->labelTotal->setFont(f);
+    ui->labelWatchAvailable->setFont(f);
+    ui->labelWatchPending->setFont(f);
+    ui->labelWatchImmature->setFont(f);
+    ui->labelWatchTotal->setFont(f);
+}