Let the theme set a button's atom gap

`Button` hard-coded `spacing.icon_spacing` as the gap between its icon and
its text. A theme that gives a button its own metrics wants to set that gap
too, so it becomes a `ButtonStyle` field, overridden by `Button::gap`.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

examples/styling_engine/src/main.rs

@@ -69,6 +69,7 @@ impl StyleProvider<ButtonStyle> for MyTheme {
                 .inner_margin(8),
             text_style: base.text,
             min_size: args.style.spacing.interact_size,
+            gap: args.style.spacing.icon_spacing,
         }
     }
 }