Add shared LNURL types, indexing helpers, and warnings.

Introduce lnurl as a first-class mint type with probe/announcement fields
and shared helpers the API and web can both rely on.

Co-authored-by: Cursor <cursoragent@cursor.com>

shared/src/index.ts

@@ -5,3 +5,5 @@ export * from './score.js';
 export * from './nuts.js';
 export * from './warnings.js';
 export * from './fedimint.js';
+export * from './lnurl.js';
+export * from './indexing.js';