Invoice Escrow: Add Grace Period Window for Overdue Invoice Settlement

[nwosum113-ui]

Summary

Introduces a configurable grace period for invoice escrow payments to handle delayed transactions or disputes.

Changes

• Adds grace_period_seconds configuration parameter to control payment completion window
• Updates error handling to account for grace period expirations
• Modifies event emissions to track grace period start/end and completion status
• Extends test coverage for grace period scenarios
• Updates contract state transitions to respect grace period logic

Impact

• Contracts: invoice-escrow module now supports delayed payment completion
• Errors: New GracePeriodExpired error variant for expired payments
• Events: New GracePeriodStarted and GracePeriodCompleted events
• Tests: Expanded test cases for grace period edge cases
• State: Payment statuses now include PendingGracePeriod state

closes #84

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[chizzy192]

Hi @nwosum113-ui, please rebase your branch on dev to resolve storage layout conflicts in storage.rs and types.rs. Confirm that grace period expiration calculations work correctly against current Soroban ledger timestamps.