fix: increase z-index of modal

[markdalgleish]

No description provided.

[linear]

RNBW-2937 Ensure modal isn't rendered below other elements

Right now the modal is rendered to a portal at the end of the document which gives it a higher default zIndex, but it means that any element with an explicit positive zIndex will render on top of it. We should provide a higher default value while ensuring that it renders beneath the WalletConnect and Coinbase modals.

We may also want to allow consumers to customize the zIndex value.

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