refactor: use ??= to make stream subscriptions idempotent

Per Copilot review feedback, use ??= instead of cancel-and-reassign
to ensure subscriptions are only created once even if initialize()
is called multiple times. This is cleaner and avoids unnecessary
cancellation of active subscriptions.

Author: prajakta128

lib/providers/board_state_provider.dart

@@ -48,8 +48,7 @@ class BoardStateProvider extends ChangeNotifier {
 
     if (configProvider.config.autoStart) {
       if (!kIsWeb && defaultTargetPlatform == TargetPlatform.android) {
-        await _usbSubscription?.cancel();
-        _usbSubscription = UsbSerial.usbEventStream?.listen(
+        _usbSubscription ??= UsbSerial.usbEventStream?.listen(
           (UsbEvent usbEvent) async {
             if (usbEvent.event == UsbEvent.ACTION_USB_ATTACHED) {
               if (_isProcessing) return;
@@ -73,8 +72,7 @@ class BoardStateProvider extends ChangeNotifier {
       }
     }
 
-    await _connectivitySubscription?.cancel();
-    _connectivitySubscription = Connectivity()
+    _connectivitySubscription ??= Connectivity()
         .onConnectivityChanged
         .listen((List<ConnectivityResult> results) {
       if (results.contains(ConnectivityResult.none)) {
@@ -134,4 +132,4 @@ class BoardStateProvider extends ChangeNotifier {
     legacyFirmwareNotifier.dispose();
     super.dispose();
   }
-}
+}