uikit: center PlAlert close button vertically

.changeset/pl-alert-close-btn-centering.md

@@ -0,0 +1,5 @@
+---
+"@milaboratories/uikit": patch
+---
+
+`PlAlert`: center the `closeable` close button on the first line of text. The X was pinned ~6px too low on single-line alerts; it now aligns with the text on both single-line and multi-line alerts.

lib/ui/uikit/src/components/PlAlert/pl-alert.scss

@@ -70,7 +70,9 @@
 
   &__close-btn {
     position: absolute;
-    top: 10px;
+    // Centre the 40px button on the first content row. The row's centre sits at
+    // 24px (12px padding + half the 24px row); top = 24 − 20 (half the button).
+    top: 4px;
     right: 12px;
   }
 }