Add comprehensive unit tests (Jest + RTL) and Cypress E2E tests — 95%+ coverage

[devin-ai-integration]

Summary

Adds a complete testing infrastructure to the ecommerce app, going from 0% to 95%+ test coverage.

What's included:

Phase 1 — Testing dependencies:

• Jest ^29, @testing-library/react ^12, @testing-library/jest-dom ^5, @testing-library/user-event ^14
• jest-environment-jsdom, identity-obj-proxy (CSS mock), @types/jest
• Cypress ^13 for E2E, next-router-mock
• New scripts: test, test:watch, test:e2e, cypress:open

Phase 2 — Jest configuration:

• jest.config.js using next/jest with CSS module mocking and coverage collection
• jest.setup.js with jest-dom matchers

Phase 3 — 17 unit test suites, 93 tests:

• context/__tests__/StateContext.test.js — all cart logic (onAdd, onRemove, toggleCartItemQuanitity, incQty, decQty, showCart, reset)
• components/__tests__/ — Footer, Product, HeroBanner, FooterBanner, Layout, Navbar, Cart
• pages/__tests__/ — index (+ getServerSideProps), success, _app
• pages/product/__tests__/slug.test.jsx — ProductDetails + getStaticPaths/getStaticProps
• pages/api/__tests__/ — stripe handler (POST, 405, error paths), hello handler
• lib/__tests__/ — client, getStripe (singleton caching), utils (runFireworks confetti)

Phase 4 — Cypress E2E tests:

• home.cy.js — hero banner, product cards, footer banner, navigation
• product-detail.cy.js — product page navigation, qty increment/decrement, add to cart
• cart.cy.js — add/remove items, qty controls, empty cart message
• checkout.cy.js — Pay with Stripe flow (API intercept)

Coverage results:

Metric	Coverage
Statements	95.51%
Branches	90%
Functions	95%
Lines	95.73%

Review & Testing Checklist for Human

• Run npm test to verify all 93 unit tests pass
• Run npm test -- --coverage to verify 95%+ coverage report
• Review the StateContext tests — these test the core cart business logic
• For E2E tests: start dev server (npm run dev), then run npx cypress run — note these require Sanity backend data or will rely on intercepts

Notes

• Used @testing-library/react@^12 (not ^14) due to React 17 peer dependency compatibility
• Used --legacy-peer-deps for npm install due to some peer dep conflicts
• Cypress E2E tests use cy.intercept() for the home page Sanity API calls; other E2E tests work against the live app
• The components/index.js barrel file shows low coverage (14%) because it's just re-exports — the actual components all have 100% coverage

Link to Devin session: https://app.devin.ai/sessions/0c4802d1afb7449dab465fa7b827bcd6
Requested by: @davidbean-hash

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[devin-ai-integration]

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