refactor: Update client to use RPC interface

BREAKING CHANGES:
- All API calls now go to POST /rpc endpoint
- Request format: { method, message, signature, params }
- Response format: { success, result } or { success: false, error }
- Simplified API methods to match RPC methods
- Removed checkUsername, added isUsernameAvailable
- Renamed postOfferAnswer to answerOffer
- Removed discoverService/discoverServices (use getService)

Changes:
- Completely refactored api.ts for RPC interface
- Updated rondevu.ts wrapper methods
- Updated rondevu-signaler.ts to use new API
- Fixed exports in index.ts

src/index.ts

@@ -16,9 +16,9 @@ export type {
 export type {
     Keypair,
     OfferRequest,
-    Offer,
     ServiceRequest,
     Service,
+    ServiceOffer,
     IceCandidate,
 } from './api.js'