fix(installer): extension update now auto-continues after pre-update backup
Two bugs in the client-side resume path made the extension update return
to the selection screen (requiring a manual re-select + re-submit) instead
of continuing after the backup:
1. The re-fire clicked the OUTER <joomla-toolbar-button> (which carries the
task attr), but the web component binds its click handler to the INNER
<button>. Clicking the wrapper did nothing. Now clicks the inner element.
2. list-selection toolbar buttons start DISABLED and only enable when
boxchecked fires a `change` event. We set boxchecked.value but never
dispatched change, so the button stayed disabled and executeTask() no-opd.
jmiller