Quanta Modal ignores className and hardcodes a 90vw width

[mliebischer]

Problem

packages/components/src/components/Modal/Modal.quanta.tsx hardcodes w-full max-w-[90vw] on the modal box and ignores any className passed by the caller: the tailwind-variants result is placed after the {...props} spread on both the ModalOverlay and the inner RACModal, so a caller-provided className is always overwritten.

Its sibling Dialog.quanta.tsx handles this correctly (it merges props.className via twMerge), so the two components behave inconsistently.

Impact

• All Quanta Modal consumers render at ~90vw width: @plone/contents' DeleteModal and UploadModal, and the History revert dialog from [CMSUI] - History route #30/Add History route (#30) #120.
• The Figma design ("Volto UI – Quanta") shows much narrower confirmation modals, e.g. for the delete confirmation — so every confirmation dialog currently deviates from the design.
• There is no way to fix this from the outside, since className is silently dropped.

Proposal

1. Merge the caller's className like Dialog.quanta does (e.g. composeRenderProps(props.className, (className, renderProps) => twMerge(modalStyles(renderProps), className))), for both the overlay and the modal element.
2. Callers (DeleteModal, UploadModal, revert dialog) then pass an appropriate width, e.g. max-w-md, to match the design.
3. Optionally discuss a narrower default width — that changes all consumers at once (bigger blast radius), so it should be a deliberate follow-up.

Acceptance criteria

• Given a Quanta Modal with className="max-w-md", when it renders, then the modal box is constrained to that width.
• Given a Quanta Modal without a className, the current default rendering is unchanged.