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

vitest.config.ts

@@ -0,0 +1,20 @@
+import path from "node:path";
+import { defineConfig } from "vitest/config";
+import react from "@vitejs/plugin-react";
+
+export default defineConfig({
+  plugins: [react()],
+  resolve: {
+    alias: {
+      "@": path.resolve(__dirname, "./src"),
+    },
+  },
+  test: {
+    environment: "jsdom",
+    setupFiles: ["./tests/setupTests.ts"],
+    globals: true,
+    coverage: {
+      reporter: ["text", "lcov"],
+    },
+  },
+});