fix(mobile): keep preview login token and asset fallback build

[DIYgod]

Description

• Ensure mobile preview login always sends x-token by falling back to ac:fallback when App Check token retrieval fails.
• Fix EAS fallback web asset generation to build from apps/mobile/web-app via pnpm --dir, with inherited stdio for clearer build diagnostics.

PR Type

• Feature
• Bugfix
• Hotfix
• Other (please describe):

Screenshots (if UI change)

N/A

Demo Video (if new feature)

N/A

Linked Issues

N/A

Additional context

Please focus review on preview login request headers and EAS fallback asset generation path.

Changelog

• I have updated the changelog/next.md with my changes.

[safedep]

SafeDep Report Summary

No dependency changes detected. Nothing to scan.

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