fix: eliminate double margin in fieldsets and p-wrapped form inputs

- Add fieldset > :last-child margin-bottom: 0 (matching article/details pattern)
- Use :nth-last-child(of :not(datalist, script, style)) to skip invisible elements
- Kill input/select/textarea margin when last visible child of <p> or <div>
  to prevent stacking with the parent's own margin-bottom

Author: Leftium

src/_forms.scss

@@ -31,6 +31,12 @@
     margin-bottom: var(#{$prefix}spacing);
   }
 
+  // When an input is the last visible child of a block that already has
+  // margin-bottom (e.g. <p>), kill the input's margin to avoid doubling.
+  :where(p, div) > :where(input, select, textarea):nth-last-child(1 of :not(datalist, script, style)) {
+    margin-bottom: 0;
+  }
+
   // ----- Focus -----
 
   :where(input, select, textarea):focus-visible {
@@ -95,6 +101,13 @@
     max-width: 100%;
   }
 
+  // Kill last-child bottom margin so it doesn't stack with fieldset padding.
+  // Uses :nth-last-child(of ...) to skip invisible elements (datalist, script).
+  :where(fieldset) > :nth-last-child(1 of :not(datalist, script, style)),
+  :where(fieldset) > :nth-last-child(1 of :not(datalist, script, style)) > :nth-last-child(1 of :not(datalist, script, style)) {
+    margin-bottom: 0;
+  }
+
   :where(legend) {
     font-weight: 600;
     padding-inline: 0.25em;