welcome: added scrollbar to privacy policy

lib/widgets/welcome_page.dart

@@ -157,6 +157,16 @@ class _WelcomePageState extends State<WelcomePage> {
       constraints: BoxConstraints(maxWidth: 460),
       child: ClipRRect(
         borderRadius: BorderRadius.circular(16),
+        child: Theme(
+          data: Theme.of(context).copyWith(
+            scrollbarTheme: ScrollbarThemeData(
+              radius: Radius.circular(16),
+              crossAxisMargin: 6,
+              mainAxisMargin: 16,
+              interactive: true,
+            ),
+          ),
+          child: Scrollbar(
             child: Markdown(
               data: terms,
               selectable: true,
@@ -168,6 +178,8 @@ class _WelcomePageState extends State<WelcomePage> {
               shrinkWrap: true,
             ),
           ),
+        ),
+      ),
     );
   }