Replace .on* event handlers with addEventListener/removeEventListener

Updated all event handler assignments to use addEventListener instead of .on* properties:
- peer/index.ts: Replaced onconnectionstatechange, ondatachannel, ontrack, onicecandidateerror
- creating-offer-state.ts: Replaced onicecandidate
- answering-state.ts: Replaced onicecandidate

Benefits:
- Proper cleanup with removeEventListener
- Prevents memory leaks by removing listeners when states/peer close
- Allows multiple listeners for the same event
- More modern and explicit event handling approach

All event handlers are now stored as class properties and properly cleaned up in cleanup()/close() methods.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/peer/answering-state.ts

@@ -6,6 +6,8 @@ import type RondevuPeer from './index.js';
  * Answering an offer and sending to server
  */
 export class AnsweringState extends PeerState {
+  private iceCandidateHandler?: (event: RTCPeerConnectionIceEvent) => void;
+
   constructor(peer: RondevuPeer) {
     super(peer);
   }
@@ -31,7 +33,7 @@ export class AnsweringState extends PeerState {
       await this.peer.offersApi.answer(offerId, answer.sdp!);
 
       // Enable trickle ICE - send candidates as they arrive
-      this.peer.pc.onicecandidate = async (event: RTCPeerConnectionIceEvent) => {
+      this.iceCandidateHandler = async (event: RTCPeerConnectionIceEvent) => {
         if (event.candidate && offerId) {
           const candidateData = event.candidate.toJSON();
           if (candidateData.candidate && candidateData.candidate !== '') {
@@ -43,6 +45,7 @@ export class AnsweringState extends PeerState {
           }
         }
       };
+      this.peer.pc.addEventListener('icecandidate', this.iceCandidateHandler);
 
       // Transition to exchanging ICE
       const { ExchangingIceState } = await import('./exchanging-ice-state.js');
@@ -53,4 +56,10 @@ export class AnsweringState extends PeerState {
       throw error;
     }
   }
+
+  cleanup(): void {
+    if (this.iceCandidateHandler) {
+      this.peer.pc.removeEventListener('icecandidate', this.iceCandidateHandler);
+    }
+  }
 }