feat(add): frontend enhancements, performance optimizations, and role-based access controls

[QueenColly]

PR Summary

This PR addresses a batch of frontend enhancements, performance optimizations, and role-based access controls to improve the overall UX and stability of the PetAd application.

What changed

• Empty State Screens (Add Empty State Screens #369)
  • Created a highly reusable <EmptyState /> component.
  • Replaced hardcoded empty states in ProfilePage, UserDashboard, and various list pages with the new standard component.
• Frontend Performance (Optimize Frontend Performance #370)
  • Implemented component-level code splitting using React.lazy() and <Suspense> for heavy modals (e.g., DocumentUploadModal, AdoptionDetailsModal).
  • Added native HTML loading="lazy" attributes to images across the application.
• Error & 404 Pages (Create 404 and Error Pages #371)
  • Designed custom NotFoundPage (404) and ErrorPage (500) views utilizing the new EmptyState component.
  • Integrated the new pages into the React Router configuration.
• Role-Based UI (Implement Role-Based UI Permissions #372)
  • Added a <RoleGuard /> component to securely conditionally render UI elements based on the current user's role.
  • Hidden administrative tools from standard users on the UserDashboardPage.
• General Fixes
  • Fixed minor syntax errors (trailing commas) and typos across several files.

Why this matters

• UX/UI: Users now have clear, beautifully designed indicators when data is empty or missing, rather than blank screens or broken layouts.
• Security: Non-admin users are no longer exposed to administrative settings/links, preventing confusion and unauthorized navigation attempts.
• Performance: Initial JS bundle sizes are significantly reduced because heavy modal components are now lazy-loaded on demand. Off-screen image lazy-loading drastically speeds up the initial page paint.

Related Issues

• Closes Add Empty State Screens #369
• Closes Optimize Frontend Performance #370
• Closes Create 404 and Error Pages #371
• Closes Implement Role-Based UI Permissions #372

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