form: add spacing between elements inside fieldsets

- common.css: fieldset > *:not(:last-child) gets margin-bottom: var(--space-xs)
- admin.css: .param-form fieldset > * zeroes margin to avoid double-spacing with flex gap

app/public/assets/css/admin.css

@@ -904,6 +904,10 @@
     gap: var(--space-xs);
 }
 
+.param-form fieldset > * {
+    margin-bottom: 0;
+}
+
 .param-form legend {
     padding: 0 var(--space-xs);
 }

app/public/assets/css/common.css

@@ -571,6 +571,10 @@ fieldset {
     margin: 0;
 }
 
+fieldset > *:not(:last-child) {
+    margin-bottom: var(--space-xs);
+}
+
 legend {
     font-size: var(--step--1);
     font-weight: 600;