Set and persist theme preference in localStorage

Implemented logic to store and retrieve theme preferences using localStorage for both logged-in users and guests. This ensures the selected theme is applied immediately on load, preventing theme flash issues. Adjusted initialization to apply the correct theme at app startup.

frontend/src/main.jsx

@@ -1,9 +1,17 @@
 import React from "react";
-import { createRoot } from "react-dom/client";
+import ReactDOM from "react-dom/client";
 import App from "./App.jsx";
+import { applyTheme, DEFAULT_THEME_KEY } from "./styles/themes";
 import { registerSW } from "virtual:pwa-register";
 
-createRoot(document.getElementById("root")).render(<App />);
+try {
+  const key = localStorage.getItem("hpTheme:guest") || DEFAULT_THEME_KEY;
+  applyTheme(key);
+} catch {
+  applyTheme(DEFAULT_THEME_KEY);
+}
+
+ReactDOM.createRoot(document.getElementById("root")).render(<App />);
 registerSW({ immediate: true });
 const updateSW = registerSW({
     immediate: true,