Every <button> inside ThemeCustomizerEnhanced was bare — no `type`
attribute, defaulting to `type="submit"`. Inside CreateEventPage's
<form onSubmit={handleSubmit}>, that turned every theme/layout/header/
divider/control/colour-mode/CSS-template click into a form submission.
When the form was empty, validation killed the submit silently — that
showed up earlier as #317.2 ("theme picker unclickable").
When the form was filled (event_name set, etc.), validation passed,
`createMutation.mutate(payload)` ran, and the user was navigated to a
freshly-created event they never asked for — #326's reported symptom.
Fix: add `type="button"` to all 9 unmarked <button>s in the customizer.
Also covered by smoke spec 09-create-event-no-instant-submit which fills
the form, clicks Modern Masonry, and asserts the URL stays on
/admin/events/new and the events count is unchanged.
Paul Nothaft
2eead52319