Enforce vendor-hosted cube images and retire staff add page

[Saterz]

Summary

• add hostname normalization utility and enforce vendor-hosted image URLs in cube form validation
• apply vendor hostname validation to submit and staff edit flows with clearer helper text
• remove the staff add cube route and its UI entry point from the manage cubes page

Testing

• npm run build (fails: missing PUBLIC_TURNSTILE_SITE_KEY in environment)

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Codex Task

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