- 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.
