Build a signup/checkout/plan-selector UI

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Problem: No signup, billing, checkout, or plan-selection UI exists anywhere in the product today.

Area: ORB / Onboarding

Proposal: Build this UI, tied to the billing data model designed in the Billing category.

Deliverables:

• A working signup/checkout/plan-selector flow.

Boundaries:

• Depends on "Design a quota/plan/subscription/invoice data model" in the Billing category above.

Design system: this is a new UI surface — it must be built on the shared design-system package from #4966/#4967, not a new one-off styling approach. Confirm the package exists and is stable before starting.

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JSO-3589

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Deferred as a group tonight (2026-07-19): this cluster (#4919, #4920, #4930, #4931, #4932) needs a real billing-provider and pricing-model decision with no engineering-inferable answer, and nothing else in ORB Cloud Readiness is currently blocked on it. Left open, revisit in a dedicated pricing/business session rather than guessed at here. #4920 (the plan/quota/subscription/invoice data model) is the right one to start with whenever that session happens — the other four build on it.

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Direction ratified (2026-07-21): signup is public (not a hand-picked allowlist) but gated behind a beta/waitlist screen — anyone can request access, the maintainer manually approves based on project/use-case fit, subject to change once volume requires something more automated. This is the middle-ground onboarding model for the pilot phase, distinct from both full self-serve and a purely maintainer-run allowlist.

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Requirement made explicit (2026-07-21): the plan-selector step needs to let a customer choose ORB, AMS/Rent-a-Loop, or both independently — not present them as one bundled purchase. Same requirement as just added to #4920, which this UI reads its plan options from.