feature: second-factor + withdrawal-limit policy

[nanaf6203-bit]

Summary

No global 2FA policy for high-value withdrawals.

Affected area

• Settings: whitelist addresses, daily withdrawal cap, mandatory 2FA above X.

Acceptance criteria

• Implement the change described above.
• Add or update unit/e2e tests locking in the new behaviour.
• Update README / docs to reflect the new state.

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Filed automatically as part of the PropChain code-review sweep.