feat: encrypted identity export/import for cross-platform key transfer

[lmanchu]

Problem

Users switching between platforms (iOS → Android, iOS → Windows) need a way to transfer their Nostr identity (nsec) without relying on platform-specific sync (iCloud).

Proposal

Add an encrypted .sigil export file format:

1. Export: User taps "Export Identity" in profile, sets a password. App generates a JSON file with scrypt-encrypted nsec + contacts + relay list.
2. Import: On new device, user opens .sigil file, enters password, identity restored.

Implementation Notes

• guard.rs already has scrypt + SHA-256 key encryption (sigil encrypt-key) — reuse for export
• File format: { "version": 1, "encrypted_nsec": "...", "contacts": [...], "relays": [...], "salt": "...", "nonce": "..." }
• iOS: share sheet export, document picker import
• CLI: sigil export-identity / sigil import-identity
• Android/Windows: same file format when those clients exist

Current Key Transfer Options

• ✅ Device Linking (QR scan between two devices)
• ✅ Manual nsec copy-paste (CLI)
• ✅ iCloud sync (iOS/Mac only, opt-in)
• ❌ This issue: encrypted file export (cross-platform)

Priority

Low — current options cover most cases. This becomes important when Android/Windows clients ship.