feat(POR-21011): propagate initial studio prompt to onboarding signup

[elhamzaouiaymen]

Summary

• When a user types a prompt in the hero or CTA composer and clicks Start building, they are redirected to the Studio onboarding signup flow instead of the generic app URL.
• The redirect includes referrer=studio and initialPrompt query params so the dashboard can prefill the onboarding experience.
• Dashboard target is environment-aware: production marketing hosts (assembly.com, www.assembly.com) go to dashboard.assembly.com; local dev, Vercel previews, and staging deploys go to dashboard.assembly-staging.com.
• Optional override via NEXT_PUBLIC_DASHBOARD_ORIGIN env var.

Closes POR-21011

Demo

https://www.loom.com/share/5f53c20147b7464a96d01ce49c81d835

Test plan

• Type a prompt on the homepage hero and click Start building — lands on /signup?referrer=studio&initialPrompt=... on the staging dashboard
• Repeat from the bottom CTA section — same redirect behavior
• Confirm production marketing host resolves to dashboard.assembly.com
• Confirm local dev / preview deploy resolves to dashboard.assembly-staging.com

[linear-code]

POR-21011

[vercel]

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Project	Deployment	Actions	Updated (UTC)
assembly-studio-website	Ready	Preview, Comment	Jul 7, 2026 3:53pm