Improve mobile detection logic to handle server-side rendering

Add a check for `window` existence in `useIsMobile` hook to prevent errors during SSR.

Replit-Commit-Author: Agent
Replit-Commit-Session-Id: f44f1d1b-1dd8-407b-8603-db12902e1a15
Replit-Commit-Checkpoint-Type: intermediate_checkpoint

src/hooks/use-mobile.tsx

@@ -6,6 +6,8 @@ export function useIsMobile() {
   const [isMobile, setIsMobile] = React.useState<boolean | undefined>(undefined);
 
   React.useEffect(() => {
+    if (typeof window === 'undefined') return;
+    
     const mql = window.matchMedia(`(max-width: ${MOBILE_BREAKPOINT - 1}px)`);
     const onChange = () => {
       setIsMobile(window.innerWidth < MOBILE_BREAKPOINT);