[BUGFIX] Fix "not focusable" error on mandatory conditional checkbox groups

[kitzberger]

In multi-step forms, a mandatory checkbox group (multiple inputs sharing name="...[]") that is conditionally hidden/shown by powermail_cond triggers a browser console error on submit when the user checks a non-first checkbox:

An invalid form control with name='...' is not focusable.

Two bugs in PowermailConditions.js cause this:

1. #getFieldByMarker uses querySelector (singular) and returns only the first checkbox of a multi-value group. #hideField/#showField therefore only disable/enable and derequire/rerequire that single checkbox, leaving the rest untouched.

2. #rerequireField unconditionally restores a native HTML5 "required" attribute, even when the field originally carried powermail's custom "data-powermail-required". Powermail uses data-powermail-required (not native required) for multi-checkbox mandatory fields and handles "at least one checked" via data-powermail-multiple. The native "required" added by #rerequireField is then enforced by reportValidity() on a field inside a display:none fieldset (hidden by the multi-step navigation), which the browser cannot focus.

Fix:

• #hideField/#showField now iterate querySelectorAll('input, select, textarea') over the wrapping container so every input in a multi-value group is disabled/enabled and derequired/rerequired.
• #derequireField/#rerequireField bookmark which required attribute was present (native "required" vs "data-powermail-required") and restore the same one, preserving powermail's custom validation for multi- checkbox groups.

Resolves: #128