Contextual form loses clicks and invalid values silently

Problem/Motivation

Found while testing UI Patterns #3622928. The contextual form saves on change through htmx. Two things go wrong around that request:

1. Clicks are lost. Clicking a button while a field has focus fires change on mousedown. The htmx request adds db-htmx-before-request to the builder, which sets pointer-events: none. Mouseup lands on the wrapper, so the click never reaches the button. The UI Patterns source selector "Save" needs two clicks.
2. Invalid values are dropped silently. The island <form> is the htmx element and has no novalidate. htmx checks browser validity first and stops the request without a message. A URL widget holding /node is never sent. Every later change sends the same invalid form, so the whole form looks blocked.

Steps to reproduce

1. Add a Bootstrap Button and select it.
2. URL prop: Configure, pick Token, type a token, click Save. The row stays open. A second click closes it.
3. Another Button: type /node in the URL field, then tab out. Nothing is updated and no error shows. Toggling "Disabled" does nothing either.

Proposed resolution

• Don't block the pointer for a request triggered by a form change.
• Add novalidate to island forms, so the server validates and the error shows as a toast.

Remaining tasks

• Playwright tests, failing first.
• Fix.