ui: Make toggle button group responsive (#36100)

This PR improves the toggle button group to be more responsive across
different layouts. This is accomplished by ensuring each button takes up
the same amount of space in the parent containers layout.

Ideally, this should be done with grids instead of a flexbox container,
as this would be much better suited for this purpose. Yet, since we lack
support for this, we go with this route for now.

| Before | After |
| --- | --- |
| <img width="1608" height="1094" alt="Bildschirmfoto 2025-08-13 um 11
24 26"
src="https://github.com/user-attachments/assets/2a4b5a59-6483-4f79-8fcb-e26e22071795"
/> | <img width="1608" height="1094" alt="Bildschirmfoto 2025-08-13 um
11 29 36"
src="https://github.com/user-attachments/assets/e6402729-6a8f-4a44-b79e-a569406edfff"
/> |


Release Notes:

- N/A

crates/onboarding/src/basics_page.rs

@@ -58,7 +58,7 @@ fn render_theme_section(tab_index: &mut isize, cx: &mut App) -> impl IntoElement
                 .tab_index(tab_index)
                 .selected_index(theme_mode as usize)
                 .style(ui::ToggleButtonGroupStyle::Outlined)
-                .button_width(rems_from_px(64.)),
+                .width(rems_from_px(3. * 64.)),
             ),
         )
         .child(
@@ -305,8 +305,8 @@ fn render_base_keymap_section(tab_index: &mut isize, cx: &mut App) -> impl IntoE
         .when_some(base_keymap, |this, base_keymap| {
             this.selected_index(base_keymap)
         })
+        .full_width()
         .tab_index(tab_index)
-        .button_width(rems_from_px(216.))
         .size(ui::ToggleButtonGroupSize::Medium)
         .style(ui::ToggleButtonGroupStyle::Outlined),
     );