test: add frontend testing infrastructure with vitest
- Introduce Vitest + React Testing Library + jsdom environment
- Add useDragSort hook unit tests covering:
* Sorting logic (sortIndex → createdAt → name)
* Successful drag operation (API call + cache invalidation)
* Failed drag operation (error toast display)
* Edge case (no valid target, no API call)
- Configure global test setup (i18n mock, auto cleanup)
- Update TypeScript configs to include tests/ directory
- Add test development plan documentation
Test Coverage:
✓ Provider drag-and-drop sorting core logic
✓ React Query cache refresh
✓ Toast notification display
✓ Boundary condition handling
Test Results: 4/4 passed (671ms)
Next Steps: Sprint 2 - component tests with MSW mock layer
Jason