feat: Enhance Nostr chat functionality and debugging

- Improve console logging in ChatComponent and useNostrChat for better tracking of message sending, user authentication, and key management.
- Update user authentication checks to ensure valid Nostr keypairs are available before sending messages.
- Refactor message handling logic to streamline subscription and processing of incoming messages, enhancing overall chat experience.

src/lib/api/lnbits.ts

@@ -40,6 +40,7 @@ interface User {
   username?: string
   email?: string
   pubkey?: string
+  prvkey?: string  // Nostr private key for user
   external_id?: string
   extensions: string[]
   wallets: Wallet[]