Add Cloudflare R2 repository setup UI

[agouny]

Summary

• add Cloudflare R2 to the repository setup storage picker
• add an R2 setup form with account ID, bucket, jurisdiction, optional endpoint, credentials, TLS flags, prefix, and session token
• add tests for the R2 form and repository setup payload

Validation

• npm test -- SetupRepositoryR2
• npm test -- SetupRepository
• npm run lint
• npm run build