refactor(ui): class update-banner buttons in MainWindow

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/ClaudeDo.Ui/Views/MainWindow.axaml

@@ -109,12 +109,12 @@
           <Run Text="{Binding UpdateBannerLatestVersion}"/>
         </TextBlock>
         <Button Grid.Column="1"
+                Classes="btn"
                 Margin="0,0,8,0"
-                Padding="10,3"
                 Content="Update now"
                 Command="{Binding UpdateNowCommand}"/>
         <Button Grid.Column="2"
-                Padding="10,3"
+                Classes="btn"
                 Content="Dismiss"
                 Command="{Binding DismissBannerCommand}"/>
       </Grid>