fix: add centering classes for toast (start-50, translate-middle-x)

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

build-bootstrap-subset.js

@@ -179,6 +179,8 @@ async function buildBootstrapSubset() {
                     'position-fixed',
                     'bottom-0',
                     'end-0',
+                    'start-50',
+                    'translate-middle-x',
                     'text-dark',
                     'bg-warning',
                     'btn-close',