chore(nextjs): rename app-router to nextjs and update to v15

[joshblack]

Update our app-router to use Next.js v15, update the example to be called "nextjs"

Changelog

New

Changed

• Move app-router example to nextjs
• Update example to use Next.js v15 and React 19

Removed

Rollout strategy

• None; if selected, include a brief description as to why

[changeset-bot]

⚠️ No Changeset found

Latest commit: e37c21e

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.

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[github-actions]

size-limit report 📦

Path	Size
packages/react/dist/browser.esm.js	104.41 KB (0%)
packages/react/dist/browser.umd.js	104.88 KB (0%)