fix(admin): center message actions with balanced cell padding

Center the operations heading and row buttons, with 20px padding on both
sides of the cell. Add a regression contract for alignment and spacing.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tests/widget-system/message-admin-contract.test.cjs

@@ -96,6 +96,9 @@ test('message row actions live only in a dedicated rightmost operations column',
     assert.match(cells[6], /data-aksrt-msg-action="delete"/);
   });
   assert.match(empty, /colspan="7"/);
+  const css = read('assets/css/admin/messages.css');
+  assert.match(css, /\.aksrt-messages-admin \.aksrt-messages-admin__table \.column-actions\s*\{[^}]*padding-left:\s*20px;[^}]*padding-right:\s*20px;[^}]*text-align:\s*center;/);
+  assert.match(css, /\.aksrt-msg-row-actions\s*\{[^}]*justify-content:\s*center;/);
 });
 
 test('details preserve full content, escape HTML and unsafe links, and never mark read', () => {