Remove back-to-top from tab order while hidden

Add visibility:hidden alongside opacity/pointer-events so keyboard
and screen-reader users can't focus the button when it's not shown.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: aclark4life

assets/css/pillow.css

@@ -737,13 +737,15 @@ section {
     font-size: 1.1rem;
     z-index: 10;
     opacity: 0;
+    visibility: hidden;
     transform: translateY(.5rem);
     pointer-events: none;
-    transition: opacity .2s ease-in-out, transform .2s ease-in-out, background-color .15s ease-in-out;
+    transition: opacity .2s ease-in-out, transform .2s ease-in-out, background-color .15s ease-in-out, visibility .2s;
 }
 
 .back-to-top.is-visible {
     opacity: 1;
+    visibility: visible;
     transform: translateY(0);
     pointer-events: auto;
 }