fix: warn when grace window is set to zero seconds

[CodedMumu]

Closes #458

• Allow 0 as a valid parseDurationInput value (changed n <= 0 → n < 0)
• Add non-blocking zeroWarning computed value; shown as an amber alert beneath the input when the entered duration equals zero
• Save button remains enabled — the warning is informational, not blocking
• Update confirmMessage to describe a zero-second grace window clearly
• Add GraceWindowSettings.test.tsx: 4 tests covering warning visibility, positive-value no-warning path, button-enabled-at-zero, and non-integer blocking error path

Pull Request

Summary

• Describe the purpose of this PR in one or two sentences.

Checklist

• Closes #__
• Tests added or updated
• ABI snapshot updated if contract sources changed (abis/ and COMEBACKHERE-contracts/)
• Screenshots attached if UI changed
• Documentation updated if needed

Notes

• For contract changes, verify ABI snapshot hygiene with make check-abi-snapshots.
• For UI changes, include screenshots or screen recordings to help reviewers.

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