Add Stripe store auth signup command

[dengjeffrey]

Summary

• Adds shopify store stripe-auth, a Stripe/Projects.dev-specific store auth command.
• Requires a Projects.dev signup JWT via --signup.
• Plumbs the signup JWT through store auth into the PKCE bootstrap.
• Appends signup=<JWT> to the Admin OAuth /admin/oauth/authorize URL so Core/Admin can process the Projects.dev store signup before Identity login / scope approval.
• Keeps the existing shopify store auth command unchanged; --signup is not available there.

Tests

• pnpm vitest run packages/store/src/cli/commands/store/auth.test.ts packages/store/src/cli/commands/store/stripe-auth.test.ts packages/store/src/cli/services/store/auth/pkce.test.ts packages/store/src/cli/services/store/auth/index.test.ts
• pnpm nx run store:type-check

Notes

• This is a POC companion to the Core Projects.dev signup handling branch.
• The JWT is forwarded only to the initial OAuth authorize URL; it is not persisted by the CLI.