Add comprehensive Vue to Next.js 16 migration plan

This plan outlines the complete migration strategy for rewriting the Vue 3 SPA
into a modern Next.js 16 application while preserving all existing C# ServiceStack
backend APIs.

Key highlights:
- 14 phases covering all aspects of migration
- Pure UI layer - all data flows through C# APIs
- Static export to MyApp/wwwroot for C# hosting
- Zustand for state management (replaces Vue reactivity)
- IndexedDB caching layer (ports existing Vue implementation)
- TypeScript with ServiceStack JsonServiceClient
- Tailwind CSS v4
- React 19 + Next.js 16 App Router

Includes detailed implementation guides for:
- Project setup and infrastructure
- ServiceStack API integration
- State management architecture
- Routing and page structure
- Component architecture
- Authentication and session management
- Real-time features and polling
- Asset management and CDN
- Build and deployment configuration
- Testing strategy
- Migration execution timeline
- Risk mitigation and rollback plans

Author: claude