Draw a thin border around each right-sidebar panel

Enable WA_StyledBackground on the three side-panel sections and apply a
class-scoped stylesheet border so each panel gets a thin outline without
the rule cascading onto child widgets.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01DZR44tA8sn4dPqDzAVXyps

src/ui/MainWindow.cpp

@@ -60,6 +60,20 @@ MainWindow::MainWindow(Simulation* sim, const std::string& configDir,
     sideLayout->addWidget(m_buildButtonGrid,       1);
     sideLayout->addWidget(m_blueprintPanel,        1);
 
+    // Draw a thin border around each of the three side-panel sections. The class
+    // scoped selectors keep the border on the panels themselves rather than
+    // cascading onto their child widgets; WA_StyledBackground lets the plain
+    // QWidget subclasses honor the stylesheet box (border/background).
+    for (QWidget* panel : { static_cast<QWidget*>(m_selectedBuildingPanel),
+                            static_cast<QWidget*>(m_buildButtonGrid),
+                            static_cast<QWidget*>(m_blueprintPanel) })
+    {
+        panel->setAttribute(Qt::WA_StyledBackground, true);
+    }
+    m_sidePanel->setStyleSheet(QStringLiteral(
+        "SelectedBuildingPanel, BuildButtonGrid, BlueprintPanel {"
+        " border: 1px solid palette(mid); }"));
+
     m_gameWorldView->setFocus();
 
     connect(qApp, &QApplication::focusChanged, this, [this](QWidget*, QWidget* newWidget) {