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

package.json

@@ -10,7 +10,9 @@
     "build:renderer": "vite build",
     "typecheck": "tsc --noEmit",
     "format": "prettier --write \"src/**/*.{js,jsx,ts,tsx,css,json}\"",
-    "format:check": "prettier --check \"src/**/*.{js,jsx,ts,tsx,css,json}\""
+    "format:check": "prettier --check \"src/**/*.{js,jsx,ts,tsx,css,json}\"",
+    "test:unit": "vitest run",
+    "test:unit:watch": "vitest watch"
   },
   "keywords": [],
   "author": "Jason Young",
@@ -21,9 +23,14 @@
     "@types/react": "^18.2.0",
     "@types/react-dom": "^18.2.0",
     "@vitejs/plugin-react": "^4.2.0",
+    "@testing-library/jest-dom": "^6.6.3",
+    "@testing-library/react": "^16.0.1",
+    "@testing-library/user-event": "^14.5.2",
+    "jsdom": "^25.0.0",
     "prettier": "^3.6.2",
     "typescript": "^5.3.0",
-    "vite": "^5.0.0"
+    "vite": "^5.0.0",
+    "vitest": "^2.0.5"
   },
   "dependencies": {
     "@codemirror/lang-javascript": "^6.2.4",