small ui improvements

app/lib/features/geckoview/features/browser/features/contextual_toolbar/presentation/toolbar_button_registry.dart

@@ -319,7 +319,7 @@ final List<ToolbarButtonDefinition> toolbarButtonRegistry = [
   ToolbarButtonDefinition(
     spec: duplicateTabToolbarButtonSpec,
     label: 'Duplicate Tab',
-    icon: MdiIcons.tabPlus,
+    icon: MdiIcons.contentDuplicate,
     builder: (scope, context, ref) {
       return scope.isPreview
           ? CloneTabButtonView(onPressed: () {}, onLongPress: () {})

app/lib/features/geckoview/features/browser/features/contextual_toolbar/presentation/widgets/contextual_toolbar.dart

@@ -117,7 +117,10 @@ class ContextualToolbarView extends StatelessWidget {
           child: Row(
             children: buttons
                 .map(
-                  (button) => SizedBox(width: _minButtonWidth, child: button),
+                  (button) => SizedBox(
+                    width: _minButtonWidth,
+                    child: Center(child: button),
+                  ),
                 )
                 .toList(),
           ),

app/lib/features/geckoview/features/browser/presentation/widgets/browser_menu_sheet.dart

@@ -817,7 +817,7 @@ class _CloneTabExpansion extends ConsumerWidget {
     return Theme(
       data: Theme.of(context).copyWith(dividerColor: Colors.transparent),
       child: ExpansionTile(
-        leading: const Icon(MdiIcons.tabPlus),
+        leading: const Icon(MdiIcons.contentDuplicate),
         title: const Text('Clone Tab'),
         children: [
           _buildSubTile(

app/lib/features/geckoview/features/browser/presentation/widgets/tab_menu.dart

@@ -358,7 +358,7 @@ class TabMenu extends HookConsumerWidget {
                   },
                 ),
             ],
-            leadingIcon: const Icon(MdiIcons.tabPlus),
+            leadingIcon: const Icon(MdiIcons.contentDuplicate),
             child: const Text('Clone Tab'),
           ),
         if (enableContainer && settings.showContainerUi)