[6.x] Avoid layout thrashing from Reka's aria-label fallback

[jasonvarga]

Reka UI's SwitchRoot and CheckboxRoot compute a fallback aria-label via document.querySelector('[for="<id>"]').innerText, on mount and again whenever their id changes. innerText forces a full synchronous layout, so on large publish forms this runs once per toggle.

On a 105-set Bard entry that was ~10.5s of the page load, and ~21s when pressing Enter near the top of the Bard field — inserting a node renumbers every following set, which changes every nested field's id.

Reka skips that computed when a truthy aria-label attribute is supplied, so we now pass the real accessible name ourselves:

• The Toggle fieldtype passes __(config.display), the same text as its <label for>.
• Checkbox/Item.vue passes label || value when no custom slot is rendered.

Measured on the same entry: Enter-at-start 21s → 1.1s, page-load innerText cost 10.5s → 0.

Radio is intentionally not covered here: Reka's Radio overrides a caller-supplied aria-label, so it needs an upstream fix.