Add an ESLint rule that forbids the real HttpClient in frontend specs

[Yicong-Huang]

Task Summary

The repo has just been swept clean of specs that wire the real HttpClient instead of HttpClientTestingModule. To stop the pattern from creeping back, add an ESLint override in frontend/.eslintrc.json that fires on *.spec.ts files and forbids importing HttpClient or HttpClientModule from @angular/common/http. The error message should redirect contributors to HttpClientTestingModule from @angular/common/http/testing, which is the convention documented in frontend/TESTING.md.

Task Type

• Testing / QA