Add comprehensive Next.js 16 migration plan

- Complete roadmap for migrating Nuxt.js/Vue 2/Vuetify to Next.js 16/React 19/Tailwind CSS v4
- Zero backend changes - uses existing C# ServiceStack APIs via JsonServiceClient
- All 20 pages and 23 components mapped with migration strategy
- Detailed API integration layer using existing DTOs
- Build output configured for ./TechStacks/wwwroot
- 6-phase approach: 10-12 weeks timeline (270-330 hours)
- Includes state management (Zustand), auth flow, routing, styling strategy

Author: claude