input.css: add flat btn component style

the layered button is too heavy to use on already layered elements such
as cards. this is also against the material design principles.

input.css

···
           dark:text-gray-100 dark:before:bg-gray-800 dark:before:border-gray-700;
         }
 
+        .btn-flat {
+          @apply relative z-10 inline-flex min-h-[30px] cursor-pointer items-center justify-center
+          bg-transparent px-2 pb-[0.2rem] text-sm text-gray-900
+          before:absolute before:inset-0 before:-z-10 before:block before:rounded
+          before:border before:border-gray-200 before:bg-white
+          before:content-[''] before:transition-all before:duration-150 before:ease-in-out
+          hover:before:bg-gray-50
+          dark:hover:before:bg-gray-700
+          focus:outline-none focus-visible:before:outline focus-visible:before:outline-2 focus-visible:before:outline-gray-400
+          disabled:cursor-not-allowed disabled:opacity-50
+          dark:text-gray-100 dark:before:bg-gray-800 dark:before:border-gray-700;
+        }
+
         .btn-create {
             @apply btn text-white
           before:bg-green-600 hover:before:bg-green-700