Add access code gate for UI access

[TimUx]

Summary

• add an access-code gate on the home page with env-driven config and localStorage unlocking
• guard the wish and donation pages behind the same code and style a prominent entry field
• document the ACCESS_CODE variable and generate it into config.js via the entrypoint
• provide a .env.example template and README note for configuring deployments quickly
• include gunicorn in the Python requirements for container runtime

Testing

• python -m py_compile app.py

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Codex Task