fix: pass mainWindow argument to showAndFocusWindow in second-instance handler

The showAndFocusWindow function signature was updated to require a
mainWindow parameter, but the call site in the second-instance handler
was not updated, causing a TypeScript compilation error that broke CI.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/main/index.ts

@@ -116,7 +116,9 @@ async function bootstrap(): Promise<void> {
 }
 
 app.on('second-instance', () => {
-  showAndFocusWindow();
+  if (mainWindow && !mainWindow.isDestroyed()) {
+    showAndFocusWindow(mainWindow);
+  }
 });
 
 app.whenReady().then(bootstrap);