Kirki < 6.3.2 - Unauthenticated Stored XSS via Registration Metadata

Description

The plugin is vulnerable to Stored Cross-Site Scripting via registration metadata. This is due to insufficient escaping in `ExceptionalElements::image_element()`, which concatenates a user-meta value straight into an `<img src="…">` attribute. This makes it possible for unauthenticated attackers to inject arbitrary web scripts that execute whenever a user accesses a page rendering a Kirki users collection whose image element is bound to one of the nine registration meta fields. Requires public user registration to be enabled and a published page carrying a `kirki-register` element, which prints the required element nonce into the public markup.