[ HumanjavaEnterprises/node-red-contrib-nostr ] fix: read private key from Node-RED credential store, not config

HIGH: privateKey was read from config (stored plaintext in flows.json)
instead of this.credentials (encrypted credential store). Now accesses
credentials properly and declares credential type in registerType.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
https://github.com/HumanjavaEnterprises/node-red-contrib-nostr/commit/075c787d74318932683422c9f18dde381041efe0