fix(a11y): wrap sort column headers in button elements

Table sort triggers were bare th elements, which are not keyboard
focusable or announced as interactive by screen readers. Replace with
button elements inside th so keyboard users can activate sorting and
get proper focus-visible ring.

Co-Authored-By: Claude <noreply@anthropic.com>

website/static/style.css

@@ -624,16 +624,25 @@ kbd {
   text-underline-offset: 0.2em;
 }
 
-th[data-sort] {
+.sort-btn {
+  background: none;
+  border: 0;
+  padding: 0;
+  color: inherit;
+  font: inherit;
   cursor: pointer;
   user-select: none;
-  transition: color 180ms ease;
 }
 
-th[data-sort]:hover {
+.sort-btn:hover {
   color: var(--accent-deep);
 }
 
+.sort-btn:focus-visible {
+  outline: 2px solid var(--accent);
+  outline-offset: 2px;
+}
+
 th[data-sort]::after {
   content: " \2193";
   opacity: 0;