feat: Beautify qr code scanner

lib/pages/new_private_chat.dart

@@ -79,9 +79,10 @@ class NewPrivateChatController extends State<NewPrivateChat> {
   void openScannerAction() async {
     final status = await Permission.camera.request();
     if (!status.isGranted) return;
-    await showDialog(
+    await showModalBottomSheet(
       context: context,
       useRootNavigator: false,
+      //useSafeArea: false,
       builder: (_) => QrScannerModal(),
     );
   }

lib/pages/qr_scanner_modal.dart

@@ -1,3 +1,4 @@
+import 'dart:async';
 import 'dart:io';
 
 import 'package:fluffychat/utils/url_launcher.dart';
@@ -33,19 +34,32 @@ class _QrScannerModalState extends State<QrScannerModal> {
         leading: IconButton(
           icon: Icon(Icons.close_outlined),
           onPressed: Navigator.of(context).pop,
+          tooltip: L10n.of(context).close,
         ),
         title: Text(L10n.of(context).scanQrCode),
       ),
-      body: QRView(
-        key: qrKey,
-        onQRViewCreated: _onQRViewCreated,
+      body: Stack(
+        children: [
+          QRView(
+            key: qrKey,
+            onQRViewCreated: _onQRViewCreated,
+            overlay: QrScannerOverlayShape(
+              borderColor: Theme.of(context).primaryColor,
+              borderRadius: 10,
+              borderLength: 30,
+              borderWidth: 8,
+            ),
+          ),
+        ],
       ),
     );
   }
 
   void _onQRViewCreated(QRViewController controller) {
     this.controller = controller;
-    controller.scannedDataStream.listen((scanData) {
+    StreamSubscription sub;
+    sub = controller.scannedDataStream.listen((scanData) {
+      sub.cancel();
       Navigator.of(context).pop();
       UrlLauncher(context, scanData.code).openMatrixToUrl();
     });