C-17236 Add EU endpoint configuration support to courier-react-native

[Gabrielgvl]

Summary

• add public CourierApiUrls presets for US and EU in the React Native wrapper
• pass apiUrls through the JS, iOS, and Android bridges for CourierClient and Courier.shared.signIn
• document the EU endpoint usage pattern in the README and auth/client docs

Testing

• ./node_modules/.bin/jest --runInBand src/__tests__/api-urls.test.tsx src/__tests__/index.test.tsx
• ./node_modules/.bin/tsc --noEmit -p tsconfig.build.json
• ./node_modules/.bin/eslint src/CourierApiUrls.tsx src/client/CourierClient.tsx src/index.tsx src/__tests__/api-urls.test.tsx

[Gabrielgvl]

Superseded by #37, which is the same branch pushed directly to the upstream repo so the PR workflows can run there normally.

[Gabrielgvl]

Closing in favor of #37.