🐛 fix: SSR hydration mismatch in mobile detection & clean up sidebar style

- Added a server-snapshot fallback (`() => false`) to `useIsMobile` to ensure
  consistent results between server-side rendering and the browser, preventing
  hydration mismatches.
- Removed a redundant ternary in `PageLayout` sidebar styles, replacing
  `isMobile ? 'fixed' : 'fixed'` with a single `'fixed'` value for clarity.

These changes improve SSR reliability and tidy up inline styles without
affecting runtime functionality.

web/src/hooks/useIsMobile.js

@@ -11,5 +11,6 @@ export const useIsMobile = () => {
       return () => mql.removeEventListener('change', callback);
     },
     () => window.matchMedia(query).matches,
+    () => false,
   );
 };