JImage: never swallow an attribute the caller needs

[romain-pm]

Part of #734. A developer must never be pushed off the component because it swallowed an attribute.

<JImage> hand-lists what it omits from ImgHTMLAttributes and drops height for good. Jahia/jahia.com's card icons take their box from the width/height HTML attributes and have no CSS rule at all, so they could not use the component and fell back to getImageProps plus a manual re-apply.

• Derive the omit from the component's own props (Omit<ImgHTMLAttributes<HTMLImageElement>, keyof JImageProps>) instead of a hand-written list, so a prop added later cannot silently eat an attribute. Next.js composes both its components this way.
• Re-declare height rather than removing it — next/image omits width/height and then adds them back with its own type.
• Rename the slot width so it does not collide with the HTML attribute (slotWidth), or add intrinsic={false}.
• An open attributes map, spread onto the <img>, accepting either a record or a function of the resolved image (({ src, width, height }) => Record<string, string>). Static values are already expressible; the function form is what an analytics attribute derived from the final src needs.
• Decide priority vs preload before release. Next.js 16 renamed priority to preload and deprecated the old name.