Prefill login username from cookie

wwwroot/app.js

@@ -34,9 +34,20 @@ function setAuthUI(isAuthed) {
   if (!isAuthed) {
     const adminCard = $("admin-card");
     if (adminCard) adminCard.classList.add("hidden");
+    const loginUser = $("login-username");
+    const cachedUser = getCookie("player_username");
+    if (loginUser && cachedUser) loginUser.value = cachedUser;
   }
 }
 
+function getCookie(name) {
+  const cookie = document.cookie
+    ?.split(";")
+    .map(c => c.trim())
+    .find(c => c.startsWith(name + "="));
+  return cookie ? decodeURIComponent(cookie.split("=").slice(1).join("=")) : null;
+}
+
 function setAuthMode(mode) {
   state.authMode = mode;
   document.querySelectorAll(".auth-form").forEach(form => {