task: v4 prep

[Hugos68]

This PR preps the v4 branch:

• Add "This website has been archived" to header.
• Configure changesets to release on the v4 branch, not main
• Configure Github actions to run actions on the v4 branch (alongside main and v3)

[changeset-bot]

⚠️ No Changeset found

Latest commit: 5ed5e9b

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 no changesets

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Project	Deployment	Actions	Updated (UTC)
www.skeleton.dev	Ready	Preview	Apr 7, 2026 2:47pm

1 Skipped Deployment

Project	Deployment	Actions	Updated (UTC)
themes.skeleton.dev	Ignored		Apr 7, 2026 2:47pm