Add emergency per-key trading freezes

[rajiabdulrahmon2020-ui]

Summary

• Add protocol-admin controls to freeze and unfreeze trading for an individual key.
• Support optional freeze reason and expiry timestamp, with expired freezes reported as inactive.
• Reject buy and sell activity on frozen keys with KeyFrozen and expose freeze status.

Closes #1051

Validation

• cargo fmt --all -- --check
• git diff --check

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