wait for backend ready signal to prepare cross-frame api

not sure if API messages should be sent in prepare(), but we should
probably wait for the ready signal before doing so

ext/js/yomichan.js

@@ -129,12 +129,12 @@ class Yomichan extends EventDispatcher {
         if (!isBackground) {
             this._api = new API(this);
 
-            this._crossFrame = new CrossFrameAPI();
-            this._crossFrame.prepare();
-
             this.sendMessage({action: 'requestBackendReadySignal'});
             await this._isBackendReadyPromise;
 
+            this._crossFrame = new CrossFrameAPI();
+            await this._crossFrame.prepare();
+
             log.on('log', this._onForwardLog.bind(this));
         }
     }