fix(frontend): check if localstorage is available before use

localStorage is not available at build time so builds fail without this check

frontend/src/context/DarkModeContext.tsx

@@ -13,11 +13,16 @@
 
 export function CustomThemeProvider({ children }: PropsWithChildren) {
   const prefersDarkMode = useMediaQuery("(prefers-color-scheme: dark)");
-  const savedDarkMode = localStorage.getItem(DARK_MODE_KEY) === "1";
-  const hasSavedDarkMode = localStorage.getItem(DARK_MODE_KEY) !== null;
+  const darkModeLocalStorage =
+    typeof localStorage !== "undefined"
+      ? localStorage.getItem(DARK_MODE_KEY)
+      : null;
+
+  const savedDarkMode = darkModeLocalStorage === "1";
+  const hasSavedDarkMode = darkModeLocalStorage !== null;
 
   const initialDarkMode = hasSavedDarkMode ? savedDarkMode : prefersDarkMode;
 
   const [darkMode, setDarkMode] = useState(initialDarkMode);
 
   function toggleDarkMode() {
@@ -18,10 +23,12 @@
 
   const initialDarkMode = hasSavedDarkMode ? savedDarkMode : prefersDarkMode;
 
   const [darkMode, setDarkMode] = useState(initialDarkMode);
 
   function toggleDarkMode() {
-    localStorage.setItem(DARK_MODE_KEY, darkMode ? "0" : "1");
+    if (typeof localStorage !== "undefined") {
+      localStorage.setItem(DARK_MODE_KEY, darkMode ? "0" : "1");
+    }
     setDarkMode(!darkMode);
   }