feat(frontend): implement SPA routing for authentication and core navigation flows

[Aqsa30nz]

Summary

Introduces client-side routing for core user flows, replacing scroll-based navigation with a structured SPA routing system.

Changes

• Added React Router-based navigation for core pages

• Introduced routes:

  • /auth → Authentication page (placeholder)
  • /listings → Listings page (placeholder)
  • /listings/new → Create listing page (placeholder)

• Updated landing page navigation:

  • "Sign in" → /auth
  • "Browse listings" → /listings
  • "List an item" → /listings/new

• Retained scroll-based navigation for informational sections:

  • How it works
  • Trust & Safety
  • Mobile app section

Behavior

Before:

• All navigation used scroll-based section jumps

After:

• Core actions use SPA routing
• Informational sections remain scroll-based for landing UX

Notes

• Auth, listings, and listing creation pages are placeholders and will be implemented in follow-up work.
• This PR focuses strictly on routing structure and navigation architecture as defined in issue [Enhancement] Implement proper SPA routing for authentication and core navigation flows #45.

Impact

• Establishes scalable routing foundation for the frontend
• Improves navigation clarity and separation between routes and sections
• Aligns application structure with standard SPA architecture