Improve firebase initialization logic

Add check to prevent fatal error on script reload.
In Firebase V9 Compat, if firebase.initializeApp() is called twice (which can happen if a mobile browser aggressively caches and re-executes the script, or if you hot-reload your local server), Firebase throws a fatal internal error: "Firebase App named '[DEFAULT]' already exists." While our silent catch block would hide this error from the participant, it's methodologically safer to prevent the error from firing at all.

Author: hashexplaindata

code/firebase-config.js

@@ -13,7 +13,10 @@ const firebaseConfig = {
 // Defensive execution to prevent UI thread blocking if the CDN fails to load
 if (typeof firebase !== 'undefined') {
     try {
-        firebase.initializeApp(firebaseConfig);
+        // Prevent fatal error if the browser aggressively reloads the script
+        if (typeof firebase.initializeApp === 'function' && (!firebase.apps || firebase.apps.length === 0)) {
+            firebase.initializeApp(firebaseConfig);
+        }
     } catch (e) {
         // Absolute silence mandated. No console.error here.
         // The failure will be naturally caught by experiment.js falling back to localStorage.