Button text alignment

Button has no text alignment option. For wide buttons, the text aligns left and cannot be centered without additional coding complexity. e.g.

```
let btn = ui.add(
Button::new(
RichText::new("Some text)
.color(Color32::WHITE),
)
.fill(Color32::DARK_GREEN)
.min_size(vec2(250.0, 20.0)),
);
```

Wrapping this in ```ui.horizontal``` does appear to center the text.

Start by locating the Button widget implementation and its existing layout or widget tests. Trace how a wide button positions its label, then add a text-alignment option that supports centering and verify that the provided wide-button example renders centered text.

Written by the indexing model from the issue text.