Fix: docs(frontend): add frontend/README.md for local dev onboarding

[brite-side0]

Closes #1668

Summary

This is a documentation-only change; no code or config files are modified. The existing frontend/README.md already contains a 'Styling conventions (CSP-safe, no inline styles)' section — preserve it and integrate the new onboarding content (overview, setup, scripts, directory structure, testing, links) around it rather than overwriting. Verify the script list matches package.json exactly, including the newer scripts (test:a11y, test:ci, test:e2e:ci, a11y:all, playwright:install, analyze, bundlewatch). Note the i18n guide currently only supports 'en'; do not list es/fr/de as supported. Do not add inline styles in any example code snippets, per the CSP rule documented in the file.

Changes

• Create frontend/README.md with an overview of the Handsoff web frontend, including a stack summary extracted from package.json (Next.js 16.2.11, React 19.2.8, TypeScript 7, Zod 4.5.4, Jest 30, Playwright 1.61, ESLint 9, Prettier 3) and Node/npm engine requirements (Node >=18, npm >=9). (frontend/README.md)
• Add a 'Getting started' section documenting prerequisites, npm install, the required NEXT_PUBLIC_API_URL environment variable (defaulting to http://localhost:3001 in tests per jest.config.js), and npm run dev to start the Next.js dev server on http://localhost:3000. (frontend/README.md)
• Add an 'Available scripts' section listing every script from package.json verbatim with a short description: generate-client, dev, build, start, lint, format, format:check, test, test:watch, test:coverage, test:a11y, test:ci, test:e2e, test:e2e:ui, test:e2e:headed, test:e2e:debug, test:e2e:chrome, test:e2e:firefox, test:e2e:safari, test:e2e:mobile, test:e2e:ci, test:e2e:report, test:all, lighthouse, axe, a11y:manual, a11y:all, playwright:install, analyze, bundlewatch. Ensure names match package.json exactly. (frontend/README.md)
• Add a 'Directory structure' section describing src/ (app router, components, lib including i18n and api schema), e2e/ (Playwright specs, with mocked vs integration split), scripts/ (run-e2e-tests.js, lighthouse-audit.js, axe-audit.js, manual-testing-checklist.js), public/, and docs/. (frontend/README.md)
• Add a 'Testing' section covering unit tests with Jest (config in jest.config.js, 80% coverage thresholds, jsdom environment, @/ path alias, e2e/ excluded) and e2e tests with Playwright (config in playwright.config.ts, mocked specs run on every PR, integration specs require a live backend and run on merge to main, staging project activated via STAGING_URL, baseURL via BASE_URL, npm run playwright:install first). Include the relevant commands. (frontend/README.md)
• Add a 'Styling conventions' note referencing the CSP-safe no-inline-styles rule already documented in the existing README content, keeping the existing sections intact and appending the new onboarding sections around them. (frontend/README.md)
• Add a 'Further reading' section linking to frontend/I18N_GUIDE.md and frontend/docs/ for additional documentation. (frontend/README.md)