fix(M-05): cache language labels in applyLanguage() -- avoids redundant API calls on rapid language switching

frontend/js/app.js

@@ -938,6 +938,10 @@ function toggleTheme() {
 // M-05 FIX: cache language labels to avoid re-fetching on every change
 const _langLabelCache = {};
 
+// M-05 FIX: cache language labels -- applyLanguage() was re-fetching on every change
+// causing multiple concurrent API calls during rapid language switching
+const _langLabelCache = {};
+
 async function applyLanguage(lang) {
   const badge = document.getElementById("lang-badge");
   if (!lang || lang === "en") {