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

tests/setupTests.ts

@@ -0,0 +1,23 @@
+import "@testing-library/jest-dom";
+import { afterEach, beforeAll } from "vitest";
+import { cleanup } from "@testing-library/react";
+import i18n from "i18next";
+import { initReactI18next } from "react-i18next";
+
+beforeAll(async () => {
+  await i18n.use(initReactI18next).init({
+    lng: "zh",
+    fallbackLng: "zh",
+    resources: {
+      zh: { translation: {} },
+      en: { translation: {} },
+    },
+    interpolation: {
+      escapeValue: false,
+    },
+  });
+});
+
+afterEach(() => {
+  cleanup();
+});