Expand Automated Testing and Coverage

**Goal**: Increase project reliability through more extensive automated and performance tests.

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### Tasks
- Set up component/UI tests (React Testing Library) for core components
- Add API client tests for critical flows (auth, CRUD, error handling)
- Write integration test covering key user journey (login, create, add endpoint, run test)
- Add simple performance benchmarks (with example test)
- Enable coverage reporting and add coverage badge to README

**References:** See [QUICK_WINS.md](https://github.com/adhit-r/aran/blob/main/QUICK_WINS.md)

**Acceptance Criteria**
- Meaningful test output and regressions detected early
- Over 30% code coverage
- Performance stability tracked
- Readme surfaces test/coverage status to all contributors

Start by reading QUICK_WINS.md and reviewing the repository's existing TypeScript and React test setup; no source files or test entry points are named in the issue. Identify how component, API-flow, integration, performance, and coverage tests are currently run, then update README with the coverage badge. Done means the requested tests and benchmarks run, coverage exceeds 30%, regressions are surfaced, and README reports test and coverage status.

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