linux: fix crash on tray Open/Settings and add Fusion style fallback

- Fix segfault when calling .first() on empty rootObjects() or
  topLevelWindows() lists in tray menu handlers
- Add Fusion as fallback Qt Quick Controls style to prevent QML load
  failures when platform theme modules (e.g., kvantum) are not installed
- Override QT_STYLE_OVERRIDE=kvantum to Fusion before QApplication init

The Fusion style is built into Qt and available on all platforms
(X11, Wayland, KDE, GNOME, etc.), ensuring the app works regardless
of the user's theme configuration.

Author: meerzulee

linux/main.cpp

@@ -547,8 +547,12 @@ public slots:
 private slots:
     void onTrayIconActivated()
     {
-        QQuickWindow *window = qobject_cast<QQuickWindow *>(
-            QGuiApplication::topLevelWindows().constFirst());
+        const auto windows = QGuiApplication::topLevelWindows();
+        if (windows.isEmpty()) {
+            loadMainModule();
+            return;
+        }
+        QQuickWindow *window = qobject_cast<QQuickWindow *>(windows.constFirst());
         if (window)
         {
             window->show();
@@ -559,26 +563,26 @@ private slots:
 
     void onOpenApp()
     {
+        if (parent->rootObjects().isEmpty()) {
+            loadMainModule();
+            return;
+        }
         QObject *rootObject = parent->rootObjects().first();
         if (rootObject) {
             QMetaObject::invokeMethod(rootObject, "reopen", Q_ARG(QVariant, "app"));
         }
-        else
-        {
-            loadMainModule();
-        }
     }
 
     void onOpenSettings()
     {
+        if (parent->rootObjects().isEmpty()) {
+            loadMainModule();
+            return;
+        }
         QObject *rootObject = parent->rootObjects().first();
         if (rootObject) {
             QMetaObject::invokeMethod(rootObject, "reopen", Q_ARG(QVariant, "settings"));
         }
-        else
-        {
-            loadMainModule();
-        }
     }
 
     void sendHandshake() {
@@ -1092,6 +1096,11 @@ private slots:
 };
 
 int main(int argc, char *argv[]) {
+    // Use Fusion style as fallback for missing theme modules
+    if (qgetenv("QT_STYLE_OVERRIDE").toLower() == "kvantum")
+        qputenv("QT_STYLE_OVERRIDE", "Fusion");
+    qputenv("QT_QUICK_CONTROLS_STYLE", "Fusion");
+
     QApplication app(argc, argv);
 
     // Handle CLI commands first (returns -1 if should continue to GUI)
@@ -1224,13 +1233,13 @@ int main(int argc, char *argv[]) {
             // Handle reopen command
             if (msg == "reopen") {
                 LOG_INFO("Reopening app window");
-                QObject *rootObject = engine.rootObjects().first();
-                if (rootObject) {
-                    QMetaObject::invokeMethod(rootObject, "reopen", Q_ARG(QVariant, "app"));
-                }
-                else
-                {
+                if (engine.rootObjects().isEmpty()) {
                     trayApp->loadMainModule();
+                } else {
+                    QObject *rootObject = engine.rootObjects().first();
+                    if (rootObject) {
+                        QMetaObject::invokeMethod(rootObject, "reopen", Q_ARG(QVariant, "app"));
+                    }
                 }
             }
             else