Add public key (npub) login support

- Backend: POST /auth/login-npub endpoint, JWT carries login method
- Backend: Enforce Lightning address lock for npub logins in claim/quote
- Frontend: LoginModal supports npub via new endpoint, clear copy
- Frontend: Track loginMethod, lock LN address editing for npub users
- Handle missing Lightning address for npub users with helpful message

Made-with: Cursor

backend/src/middleware/nip98.ts

@@ -8,6 +8,7 @@ const AUTH_SCHEME = "Nostr ";
 export interface NostrAuthPayload {
   pubkey: string;
   eventId: string;
+  method?: "nip98" | "npub";
 }
 
 declare global {