fix(desktop): toast action is a real button, not a hairline text link

The notification action (`NotificationItem`) rendered as
`variant="textStrong" size="xs"` — an 11px underlined muted-grey text link
with a ~44x20px hit target. On the data-training confirm toast raised by
`surfaceModelSwitchConfirm` / `confirmModelWarning` (e.g. picking
`muse-spark-1.2-contributor`) it read as a footnote, not the one action
the toast exists for, and users reported not being able to "press to
accept".

Promote it to the SDK's `default` variant at `size="sm"`: a filled
primary button, larger hit target, obvious affordance. No new styles.

Salvaged from #96562 (toast half only). Refs #96563.

apps/desktop/src/components/notifications.tsx

@@ -236,9 +236,9 @@ function NotificationItem({ notification }: { notification: AppNotification }) {
                 notification.action?.onClick()
                 dismissNotification(notification.id)
               }}
-              size="xs"
+              size="sm"
               type="button"
-              variant="textStrong"
+              variant="default"
             >
               {notification.action.label}
             </Button>