feat(ui): add Merge button to DiffModal

src/ClaudeDo.Ui/Views/Modals/DiffModalView.axaml

@@ -63,12 +63,15 @@
                      FontFamily="{StaticResource MonoFamily}"
                      FontSize="12"
                      Foreground="{StaticResource TextDimBrush}"/>
-          <Button Grid.Column="1"
-                  Classes="icon-btn"
+          <StackPanel Grid.Column="1" Orientation="Horizontal" Spacing="4" VerticalAlignment="Center">
+            <Button Content="Merge…"
+                    Command="{Binding MergeCommand}"
+                    Margin="0,0,4,0" />
+            <Button Classes="icon-btn"
                     Content="✕"
                     FontSize="12"
-                  Command="{Binding CloseCommand}"
-                  VerticalAlignment="Center"/>
+                    Command="{Binding CloseCommand}" />
+          </StackPanel>
         </Grid>
       </Border>