Test the Manage Tasks Use Case (Hilla)

Background

The project tracker application is built with Hilla: a React/TypeScript view at src/main/frontend/views/tasks.tsx calls a @BrowserCallable Java service (src/main/java/com/example/tasks/TaskService.java) through the generated TypeScript client. The use case is specified in docs/use-cases/UC-060-manage-tasks.md. The project already contains the @UseCase annotation type at src/main/java/com/example/app/usecase/UseCase.java and has Vitest configured for the frontend.

What to Produce

Write tests for UC-060 on both layers:

1. Frontend — a Vitest + React Testing Library suite for the tasks view. Mock the generated TaskService client so no server is needed. Cover the main success scenario (tasks are listed on load, a new task can be created) and alternative flow A1 (duplicate title — the endpoint rejects and the error is shown to the user).
2. Backend — a Spring Boot integration test class that calls the TaskService bean directly against the test database. Cover the main success scenario and alternative flow A1, and keep the database consistent between tests.

Place the frontend suite in the frontend tests directory and the backend class in src/test/java/com/example/tasks/. Deliver only the test sources; no build files or configuration changes are needed.