GristWSConnection: use settings instead of "window" object

The reference to "window" would unexpectedly break Node-based tests
where a client stays connected long enough to send a heartbeat.

GristWSSettings.getPageUrl seems explicitly made for this.

app/client/components/GristWSConnection.ts

@@ -317,7 +317,7 @@ export class GristWSConnection extends Disposable {
   private _sendHeartbeat() {
     this.send(JSON.stringify({
       beat: 'alive',
-      url: G.window.location.href,
+      url: this._settings.getPageUrl(),
       docId: this._assignmentId,
     }));
   }