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

tsconfig.json

@@ -19,6 +19,6 @@
       "@/*": ["src/*"]
     }
   },
-  "include": ["src/**/*"],
+  "include": ["src/**/*", "tests/**/*"],
   "references": [{ "path": "./tsconfig.node.json" }]
 }