style: hide back-to-top label on phones

On phones the "Top ↑" header button was wider than the row arrows, so it set the arrow column's width and left a wide gap between the downloads number and the arrow.

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

website/static/style.css

@@ -1906,7 +1906,9 @@ th[data-sort].sort-asc::after {
 
   .col-num,
   .col-cat,
-  .group-count {
+  .group-count,
+  /* Leaves the arrow column as narrow as the row arrows */
+  .back-to-top-label {
     display: none;
   }
 

website/templates/_projects_table.html

@@ -53,7 +53,7 @@
           <th class="col-cat">Tags</th>
           <th class="col-arrow">
             <button class="back-to-top" aria-label="Back to top">
-              Top &uarr;
+              <span class="back-to-top-label">Top</span> &uarr;
             </button>
           </th>
         </tr>