docs: clarify Next.js React peer support

[yamcodes]

Summary

• Rewrite the existing changeset as a consumer-facing release note
• Explain support for React 18.2.0+ in React 18 and all React 19 releases
• Include a concise installation example

This PR targets the dev branch.

[changeset-bot]

⚠️ No Changeset found

Latest commit: 1d450bd

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

This PR includes changesets to release 1 package

Name	Type
@arkenv/nextjs	Patch

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[pullfrog]

✅ No new issues found.

Reviewed changes

• Rewrote the changeset summary in .changeset/widen-nextjs-react-peer.md from an internal implementation note to a consumer-facing release note: @arkenv/nextjs now works with React 18.2.0+ (React 18 line) and every React 19 release, plus a short install example.

Verified the new wording against the codebase: the peer range in packages/nextjs/package.json is ^18.2.0 || ^19.0.0, and the "requiring React 19.2.5" claim matches the root cause documented in #1833. Docs-only change, no concerns.

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