Add focus state to button mixin for accessibility

- Add :focus pseudo-class to button mixin with outline styling
- Improves keyboard navigation accessibility for all buttons
- Uses currentColor for outline to adapt to different button styles
- Meets WCAG accessibility standards for focus visibility
- Update CSS source map after rebuild

src/scss/_mixins.scss

@@ -30,6 +30,11 @@
     transform: translateY(-1px);
   }
   
+  &:focus {
+    outline: 2px solid currentColor;
+    outline-offset: 2px;
+  }
+  
   &:active {
     transform: translateY(0);
   }