fix: restore vertical spacing for native button elements

Re-add margin-block-end to <button> and button-type <input> elements
without affecting [role="button"] on non-button elements (e.g. logo).

Commit 52cf801 removed all button margin (issue #6) but never replaced
the vertical spacing, causing wrapped buttons to collapse together.
Scoped to native elements only to avoid shifting @leftium/logo's
overlay-aligned div[role="button"].

Author: Leftium

src/_buttons.scss

@@ -30,6 +30,12 @@
     transition: background-color 0.2s, border-color 0.2s;
   }
 
+  // Vertical rhythm for flow-level buttons (not [role="button"] on
+  // arbitrary elements like logo containers — those opt in manually).
+  :where(button, [type="submit"], [type="reset"], [type="button"]) {
+    margin-block-end: 0.25em;
+  }
+
   :where(button, [type="submit"], [type="reset"], [type="button"], [role="button"]):hover {
     background-color: var(#{$prefix}primary-hover);
     border-color: var(#{$prefix}primary-hover);