feat: make getNonce optional in AuthenticationAdapter

[2wheeh]

Description

Follow-up to #2633.

... nonces must come from the server. A client-generated nonce provides no replay protection because an attacker can reuse the signed message with the same nonce.
SIWE security considerations

#2633 enabled server-side SIWE message generation, but the adapter still required getNonce. dApps following that pattern end up shipping dead boilerplate — a no-op /api/nonce endpoint or getNonce: async () => 'nonce' — just to satisfy the type, with no real-world signal flowing through it.

This PR makes getNonce optional. When omitted, the client-side nonce pre-fetch is skipped and createMessage receives only { address, chainId }.

API

AuthenticationAdapter becomes a discriminated union — fully backwards compatible:

• With getNonce — createMessage({ nonce, address, chainId }). Unchanged.
• Without getNonce — createMessage({ address, chainId }). Pre-fetch skipped; button enabled immediately.

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[changeset-bot]

🦋 Changeset detected

Latest commit: df43ae1

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 18 packages

Name	Type
@rainbow-me/rainbowkit	Patch
example	Patch
@rainbow-me/rainbow-button	Patch
rainbowkit-next-app	Patch
site	Patch
with-create-react-app	Patch
with-next-app-i18n	Patch
with-next-app	Patch
with-next-custom-button	Patch
with-next-mint-nft	Patch
with-next-siwe-iron-session	Patch
with-next-siwe-next-auth	Patch
with-next-wallet-button	Patch
with-next	Patch
with-react-router	Patch
with-remix	Patch
with-vite	Patch
with-next-rainbow-button	Patch

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