confirmation.html: center button text in action row

The .btn class uses display:inline-flex with align-items:center but
no justify-content, so text stuck to the left when the parent flex
stretched the buttons to fill the row. Add justify-content:center
scoped to the confirmation card actions.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

confirmation.html

@@ -96,6 +96,8 @@
     }
     .confirmation-card .actions .btn {
       flex: 1 1 200px;
+      justify-content: center;
+      text-align: center;
     }
     .confirmation-card .loader {
       display: inline-block;