fix(form): remove left border-radius from buttons inside form-group

The `.form-group` CSS reset `border-radius: 0` only for `.form-input`
and `.form-select`, leaving `.btn` elements with their default radius
on all sides. This caused buttons inside input groups to have rounded
left corners instead of flush edges against adjacent inputs.

src/ui/form.css

@@ -350,7 +350,8 @@
 }
 
 .form-group > .form-input,
-.form-group > .form-select {
+.form-group > .form-select,
+.form-group > .btn {
   border-radius: 0;
 }