[Lisa18289]

Adds a unit prop to NumberField for units Intl.NumberFormat does not know, like MiB or GiB. It renders 512 MiB exactly where formatOptions units appear.

Until now the fallback was writing the unit into the label ("Storage (MiB)"). The prop is better for sighted users and equal for screen reader users:

• The unit sits next to the number it belongs to, not in the label above the field.
• Custom and Intl units look the same, so a form mixing GB and MiB fields stays consistent.
• The label stays the plain field name.
• Screen readers get the same result as before: the unit is part of the field's name ("Storage MiB").

React-aria's number parser rejects unknown text in the input on every keystroke, which is why #1041 was dropped. So the unit is not part of the input value: an overlay mirrors the input box and an invisible copy of the input text places the unit right behind the number. The value stays a plain number (stepper, validation, form submit unchanged).

For screen readers the unit is attached via aria-labelledby. As with the label fallback, react-aria's step announcement says only the number ("513"). Unlike the fallback, the stepper buttons keep the label-only name, because react-aria labels them by the label id alone.

Closes #2579

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