Contract: add get_expiring_attestations() for subjects to query their own expiring credentials

[Haroldwonder]

Summary

Subjects have no way to query which of their own attestations are expiring soon without fetching all attestations and filtering client-side. This is needed for wallet UIs that want to show renewal reminders.

Proposed API

fn get_expiring_attestations(env: Env, subject: Address, within_days: u32, start: u32, limit: u32) -> Vec<Attestation>

Acceptance Criteria

• Returns attestations expiring within within_days days, sorted by expiration ascending.
• Excludes already-revoked and already-expired attestations.
• TypeScript SDK exposes getExpiringAttestations(subject, withinDays, start, limit).
• React app UserPanel uses this to show a renewal reminder.

[HushLuxe]

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