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

@@ -1171,6 +1171,7 @@ int main(int argc, char *argv[]) {
 
     QObject::connect(&server, &QLocalServer::newConnection, [&]() {
         QLocalSocket* socket = server.nextPendingConnection();
+        QObject::connect(socket, &QLocalSocket::disconnected, socket, &QLocalSocket::deleteLater);
 
         QObject::connect(socket, &QLocalSocket::readyRead, [socket, &engine, trayApp]() {
             QString msg = QString::fromUtf8(socket->readAll());
@@ -1191,8 +1192,11 @@ int main(int argc, char *argv[]) {
                 }
                 else if (msg.startsWith("cli:set-noise-mode:")) {
                     QString modeStr = msg.mid(QString("cli:set-noise-mode:").length());
-                    int mode = modeStr.toInt();
-                    if (!trayApp->areAirpodsConnected()) {
+                    bool ok;
+                    int mode = modeStr.toInt(&ok);
+                    if (!ok || mode < 0 || mode > 3) {
+                        response = "Error: Invalid noise mode";
+                    } else if (!trayApp->areAirpodsConnected()) {
                         response = "Error: AirPods not connected";
                     } else {
                         trayApp->setNoiseControlModeInt(mode);
@@ -1201,18 +1205,22 @@ int main(int argc, char *argv[]) {
                 }
                 else if (msg.startsWith("cli:set-ca:")) {
                     QString stateStr = msg.mid(QString("cli:set-ca:").length());
-                    bool enabled = (stateStr == "1");
-                    if (!trayApp->areAirpodsConnected()) {
+                    if (stateStr != "0" && stateStr != "1") {
+                        response = "Error: Invalid state";
+                    } else if (!trayApp->areAirpodsConnected()) {
                         response = "Error: AirPods not connected";
                     } else {
-                        trayApp->setConversationalAwareness(enabled);
+                        trayApp->setConversationalAwareness(stateStr == "1");
                         response = "OK";
                     }
                 }
                 else if (msg.startsWith("cli:set-adaptive-level:")) {
                     QString levelStr = msg.mid(QString("cli:set-adaptive-level:").length());
-                    int level = levelStr.toInt();
-                    if (!trayApp->areAirpodsConnected()) {
+                    bool ok;
+                    int level = levelStr.toInt(&ok);
+                    if (!ok || level < 0 || level > 100) {
+                        response = "Error: Invalid level (0-100)";
+                    } else if (!trayApp->areAirpodsConnected()) {
                         response = "Error: AirPods not connected";
                     } else {
                         trayApp->setAdaptiveNoiseLevel(level);