Add :focus-visible style to buttons (go-gitea#31799)

Buttons now show a focus style via
[`:focus-visible`](https://developer.mozilla.org/en-US/docs/Web/CSS/:focus-visible)
when the browser deems the focus to be important, like for example when
the button is focused via keyboard navigation.

<img width="492" alt="Screenshot 2024-08-07 at 22 12 51"
src="https://github.com/user-attachments/assets/060568b1-1599-4c56-bafb-b36ebb1bec35">
<img width="479" alt="image"
src="https://github.com/user-attachments/assets/885f4e10-f496-47f0-8ae5-45827ded09f8">

Author: silverwind

web_src/css/modules/button.css

@@ -24,6 +24,10 @@
   white-space: nowrap;
 }
 
+.ui.button:focus-visible {
+  box-shadow: inset 0 0 0 2px currentcolor;
+}
+
 @media (max-width: 767.98px) {
   .ui.button {
     white-space: normal;