Add floating action button to App component

Updated the `App` component in `app.tsx` to include a new floating action button (Fab) linking to Digital Data. The button now features a transparent background and no box shadow, along with an image for improved visual representation. The `useScrollToTop` function remains unchanged but has a more streamlined implementation.

src/client/dd-hub-react/src/app.tsx

@@ -22,8 +22,9 @@ export default function App({ children }: AppProps) {
   const ddButton = () => (
     <Fab
       size="medium"
-      aria-label="Github"
-      href="https://github.com/minimal-ui-kit/material-kit-react"
+      aria-label="Digital Data"
+      href="https://digitaldata.works/"
+      target="_blank"
       sx={{
         zIndex: 9,
         right: 20,
@@ -31,10 +32,14 @@ export default function App({ children }: AppProps) {
         width: 48,
         height: 48,
         position: 'fixed',
-        bgcolor: 'grey.800',
+        bgcolor: 'transparent',
+        boxShadow: 'none'
       }}
     >
-      <Iconify width={24} icon="socials:github" sx={{ '--color': 'white' }} />
+      <img
+        src="/assets/images/product/product-default.webp"
+        alt="Digital Data"
+      />
     </Fab>
   );
 
@@ -56,4 +61,4 @@ function useScrollToTop() {
   }, [pathname]);
 
   return null;
-}
+}