feat: add remote Step Plan login

[liuedcson]

Summary

• replace the loopback callback flow with developer-center init and polling
• support domestic and overseas Step Plan profiles with browser launch and a complete URL fallback
• keep poll tokens out of URLs and storage, bound network responses, and remove the legacy callback server
• add --no-browser for SSH and container environments

Validation

• pnpm run check
• pnpm test
• pnpm run build:offline
• production smoke tests for domestic and overseas approval flows