fix: Linux menu bar renders as horizontal tool buttons instead of collapsed extension popup

On Linux, QMenuBar inside a custom title bar widget (setMenuWidget) collapses
all items into the extension overflow popup. Replace with QToolButton widgets
on Linux that share the same QMenu objects. Includes hover-to-switch behavior
via event filter on open menus.

Windows and macOS paths are unchanged — guarded by #ifdef __linux__ and
runtime m_useToolButtons flag.

src/main.cpp

@@ -615,6 +615,7 @@ MainWindow::MainWindow(QWidget* parent) : QMainWindow(parent) {
     createSymbolsDock();
 
     createMenus();
+    if (m_titleBar) m_titleBar->finalizeMenuBar();
     createStatusBar();
 
     // Eliminate gap between central widget and status bar