Add support for NIP-46 "Nostr Bunker"

[Flaxscrip]

Popular implementations:

• Amber (Android) — Dedicated event signer app, widely used
• nsec.app — Browser-based remote signer
• Various other bunker implementations supporting the NIP-46 protocol

Keymaster supporting NIP-46 bunker auth would be a non-custodial, decentralized nostr key management system. Since Keymaster already derives the Nostr keypair from the DID mnemonic:

1. ✅ You already have the nsec/npub keypair
2. ✅ NIP-46 is just signing event requests and sending responses via relay
3. ✅ This would let Keymaster act as a remote signer for any NIP-46 compatible Nostr client
4. ✅ Non-custodial (keys never leave user control) vs openbunker's custodial model

The value proposition: Archon DID → derive Nostr identity → use Keymaster as your personal NIP-46 bunker for all Nostr apps. Clean, decentralized, identity-rooted.

This would significantly expand Keymaster's utility in the Nostr ecosystem. Worth exploring.