"Enhance loading experience and optimize theme application

Added a fallback background and a preload lock for smoother loading transitions. Improved theme application by applying it prior to React rendering and removed theme flash. Adjusted Service Worker registration for better performance and reliability."

frontend/src/main.jsx

@@ -4,6 +4,7 @@ import App from "./App.jsx";
 import { applyTheme, DEFAULT_THEME_KEY } from "./styles/themes";
 import { registerSW } from "virtual:pwa-register";
 
+// ✅ Theme VOR React setzen (kein Theme-Flash)
 try {
   const key = localStorage.getItem("hpTheme:guest") || DEFAULT_THEME_KEY;
   applyTheme(key);
@@ -11,12 +12,17 @@ try {
   applyTheme(DEFAULT_THEME_KEY);
 }
 
+// ✅ Preload Unlock (nach Theme!)
+document.body.classList.remove("preload");
+document.body.classList.add("ready");
+
 ReactDOM.createRoot(document.getElementById("root")).render(<App />);
-registerSW({ immediate: true });
+
+// ✅ Service Worker NUR EINMAL registrieren
 const updateSW = registerSW({
-    immediate: true,
-    onNeedRefresh() {
-      updateSW(true); // sofort neue Version aktivieren
-      window.location.reload();
-    },
-  });
+  immediate: true,
+  onNeedRefresh() {
+    updateSW(true);
+    window.location.reload();
+  },
+});