Add comprehensive backend/frontend testing framework with route-outcome coverage

[AZ0228]

Summary

• add backend route-outcome tests that execute real route handlers against an in-memory MongoDB instance (mongodb-memory-server)
• add backend test harness helpers for ephemeral DB setup/teardown and update Jest config for broader coverage and stable Node execution
• add frontend API request-lifecycle tests for postRequest to validate retry/refresh/error behavior
• upgrade CI actions to Node 24-ready versions (actions/checkout@v6, actions/setup-node@v6, actions/upload-artifact@v6)
• split CI into distinct GitHub checks/jobs for Build, Unit tests, and Coverage
• avoid false-negative build check failures from warnings by running the frontend build step with CI=false while preserving separate unit/coverage quality gates
• publish a per-run GitHub Actions step summary with backend/frontend test counts and coverage percentages
• upload richer CI artifacts (test result JSON + coverage summaries + HTML coverage reports) for backend and frontend
• expand testing documentation to include route-outcome patterns and API-lifecycle test guidance

Backend route-outcome coverage

• POST /register success path, duplicate-email conflict, profanity rejection, invite-email mismatch handling
• POST /check-username taken/available outcomes with real JWT auth middleware and real persisted test users

Testing

• npm --prefix backend run test:unit
• npm --prefix backend run test:integration
• npm --prefix backend run test:routes
• npm --prefix frontend run test:ci
• CI=true npm --prefix frontend run build (fails because warnings are treated as errors)
• CI=false npm --prefix frontend run build (passes; build output produced)
• npm --prefix backend run test:coverage -- --json --outputFile=coverage/test-results.json
• npm --prefix frontend run test:coverage -- --json --outputFile=coverage/test-results.json --coverageReporters=text --coverageReporters=lcov --coverageReporters=clover --coverageReporters=json-summary
• yq '.' .github/workflows/ci.yml (workflow parse validation)