Fix button styling for 'add' page

src/main/resources/static/add.css

@@ -66,6 +66,7 @@ label:has(#is-dataset) {
     box-sizing: border-box;
     background-color: var(--text-color);
     transition: inset-inline ease-out 50ms;
+    filter: drop-shadow(rgba(0, 0, 0, .8) 0 0 .25rem);
 }
 
 #is-dataset:not(:checked) + #is-dataset-toggle::before {

src/main/resources/static/main.css

@@ -147,10 +147,14 @@ header {
     filter: brightness(1.3);
 }
 
-.btn:is(:hover, :focus-visible) {
+.btn.flat {
+    transition: filter ease-out 50ms;
+}
+
+.btn.flat:is(:hover, :focus-visible) {
     filter: brightness(1.5);
 }
 
-.btn:active {
+.btn.flat:active {
     filter: brightness(1.75);
 }