Align Vercel deployment config and split app envs

[nighca]

Background

PR #3298 left a follow-up note on spx-gui/.env.production.cn about two remaining tasks:

1. Vercel deployment for app xbuilder should use the committed configuration instead of the environment variables configured on Vercel.
2. The env used by app xbuilder and app account should be split.

This issue tracks that follow-up work.

What this issue is about

Align deployment and runtime configuration so xbuilder uses the repository-managed config in Vercel, and separate the environment definitions for app xbuilder and app account.

Plan

• review the current config flow for spx-gui/.env.production.cn
• update Vercel deployment behavior for app xbuilder to consume the committed config
• split the env configuration used by app xbuilder and app account
• verify the deployment/runtime behavior after the split

Reference

• PR comment: Build config for mainland China deployment in release workflow #3298 (comment)