Priority: P0 (CRITICAL)

Implementation Guidelines

Structure: Use the app/ directory. Define routes with app/dashboard/layout.tsx returning {children}; shared UI nests inside app/layout.tsx automatically. Handle states with loading.tsx, error.tsx, and not-found.tsx.
Segments: Organize features with Route Groups (brackets (auth)) to be excluded from URL path. Use Dynamic Routes (brackets [slug]) and define static paths via generateStaticParams.
Specialized: Use Parallel Routes (@modal) by adding the slot to the parent layout and providing a default.tsx fallback. Use Intercepting Routes ((.)route) for advanced layouts like dashboards.

Next.js 15+ Async: Always await the params: Promise, searchParams, cookies(), and headers().
Security: Use middleware.ts for edge-side authentication and redirection. Ensure all Route Handlers (route.ts) are secured with appropriate auth checks.
RSC: Default to React Server Components (RSC). Only use 'use client' at leaf nodes for interactivity (hooks/events).
Error Boundaries: Create app/dashboard/loading.tsx to auto-wrap routes in a Suspense boundary. In error.tsx, use 'use client' and provide a reset: () => void function. const theme = cookieStore.get('theme');