Fix: Visuals::interact_cursor support in Button (#7986)

Closes #7947

## Problem

`Visuals::interact_cursor` stopped working for buttons after the
`AtomLayout` refactor in commit 6eb7bb6e. Setting `interact_cursor` to
e.g. `CursorIcon::PointingHand` no longer changes the cursor when
hovering over a `Button`.

## Root Cause

When `Button` was rewritten to use `AtomLayout` in #5830, the
cursor-override block at the end of the old `Button::ui` was not carried
over to the new `Button::atom_ui` method.

The old code had:
```rust
if let Some(cursor) = ui.visuals().interact_cursor {
    if response.hovered() {
        ui.ctx().set_cursor_icon(cursor);
    }
}
```

This was the only place `interact_cursor` was checked, so the setting
became entirely non-functional.

## Fix

Re-add the same `interact_cursor` check in `Button::atom_ui`, right
after painting and before `widget_info`, matching the original behavior.

---------

Co-authored-by: easonysliu <easonysliu@tencent.com>
Co-authored-by: Emil Ernerfeldt <emil.ernerfeldt@gmail.com>

crates/egui/src/widgets/button.rs

@@ -371,6 +371,12 @@ impl<'a> Button<'a> {
             AtomLayoutResponse::empty(prepared.response)
         };
 
+        if let Some(cursor) = ui.visuals().interact_cursor
+            && response.response.hovered()
+        {
+            ui.ctx().set_cursor_icon(cursor);
+        }
+
         response.response.widget_info(|| {
             if let Some(text) = &text {
                 WidgetInfo::labeled(WidgetType::Button, ui.is_enabled(), text)