feat: let Cap Pro buyers choose checkout currency

[richiemcilroy]

What changed

• Added a compact Cap Pro currency picker before Stripe Checkout.
• Automatic keeps Stripe localization; a manual choice sets the Checkout Session currency.
• The picker shows only currencies configured on the selected plan price. Account and guest web checkout both use the choice, while mobile guest checkout keeps its direct flow.

Validation

• 13 focused checkout tests passed, including account, guest, annual currency availability, rendered picker, and mobile regression checks.
• Scoped Biome and TypeScript checks passed; the isolated seven-file diff passed git diff --check.
• Test Stripe prices were read to confirm that monthly supports USD, GBP, and EUR, while the current test annual price supports USD.

Limits

• Production choices are resolved from the live Stripe price at runtime. Live browser replay and a screenshot were unavailable because the existing local web server was down.
• The repository-wide web type check is blocked by stale generated types for unrelated editor routes; the touched path passed a direct TypeScript check.

Confidence Score: 5/5

The PR appears safe to merge, with no outstanding findings after the latest fixes.

Summary

This PR adds a Cap Pro currency-selection step before Stripe Checkout.

• Resolves available currencies dynamically from the selected Stripe price.
• Carries manual currency choices through account and guest web checkout while retaining Stripe-controlled localization for Automatic.
• Preserves the direct mobile guest checkout path.
• Migrates both modified billing endpoints to the repository-required Effect HttpApi and apiToHandler pattern.
• Adds focused coverage for account, guest, annual-plan, sign-in-resume, picker rendering, and mobile behavior.

_{Reviews (4) · Last reviewed commit: "fix: honor selected billing currency at ..."}

[richiemcilroy]

hey @greptileai, please re-review the PR

[richiemcilroy]

hey @greptileai, please re-review the PR

[richiemcilroy]

hey @greptileai, please re-review the PR