feat: custom 404 page and global loading system

[yusuftomilola]

Summary

• Redesigned the 404 page with proper styling using the project's design system (accent/primary colors, Poppins font, fade-in
  animation) and added both "Go Back Home" and "Go Back" navigation buttons
• Created GlobalLoadingWrapper — a central loading component that shows a Spinner overlay during route transitions (via
  useNavigation) and async Redux actions (watches auth, campaigns, donations, dashboard loading states)
• Converted main pages to React.lazy() and wrapped Routes with Suspense, using the existing Spinner as the fallback to
  eliminate blank screens during code-splitting

closes #38