feat(testing): add Stryker mutation testing for frontend and backend

[EmperorNexus]

Overview

This PR adds Stryker mutation testing infrastructure to validate the effectiveness of our test suite across both frontend (React Native) and backend (Node.js/Express) modules. Mutation testing injects small changes (mutants) into source code and verifies that tests detect them, surfacing weak or missing test coverage.

Related Issue

Closes #715

Changes

🧬 Mutation Testing Infrastructure

• [ENHANCE] stryker.conf.json — frontend Stryker config with broader mutate exclusions (tests, mocks, fixtures, declarations), dashboard reporter, concurrency 4, 60s timeout, and explicit plugin declarations
• [ADD] stryker.backend.conf.json — backend Stryker config targeting backend/**/*.ts with Jest backend config and backend/tsconfig.json
• [ADD] scripts/check-mutation-score.js — mutation score validator that reads Stryker reports, enforces break threshold (50%), flags weak tests (<60%), and generates dashboard JSON
• [ADD] reports/mutation-dashboard/index.html — live web dashboard for visualizing mutation scores, thresholds, and weak test detection

📦 Package Scripts

• [ADD] mutation:frontend — run Stryker on frontend source
• [ADD] mutation:backend — run Stryker on backend source
• [ADD] mutation:all — run both frontend and backend mutation tests
• [ADD] mutation:score — check mutation scores from latest reports

🔄 CI Integration

• [ADD] mutation-frontend job — runs Stryker on src/, uploads HTML report as artifact, enforces score threshold
• [ADD] mutation-backend job — runs Stryker on backend/, uploads HTML report as artifact, enforces score threshold
• [ADD] mutation-summary job — downloads both reports, generates dashboard JSON, prints combined summary

📚 Documentation

• [UPDATE] docs/mutation-testing.md — comprehensive guide covering configuration, running tests, reports, CI, weak test detection, best practices, and troubleshooting

Verification Results

npm run lint        ✅ Pass (only pre-existing warnings)
npx tsc --noEmit   ✅ TypeScript compiles clean
npm test           ✅ 708/712 tests pass (4 pre-existing failures unrelated)

Acceptance Criteria	Status
Stryker configuration for React Native	✅ stryker.conf.json enhanced
Stryker configuration for backend	✅ stryker.backend.conf.json added
Mutation score tracking	✅ Score thresholds (high: 80%, low: 60%, break: 50%)
Mutation testing in CI	✅ 3 CI jobs in ci.yml
Weak test detection and reporting	✅ scripts/check-mutation-score.js with <60% flagging
Mutation testing dashboard	✅ reports/mutation-dashboard/index.html
Mutation testing documentation	✅ docs/mutation-testing.md updated

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