Add ESLint/Prettier config, TTS env test coverage, and missing a11y:manual script (#1305, #1306, #1307, #1308)

[Joel234-png]

Summary

• [Frontend Scaffolding] Set up ESLint and Prettier #1305: no eslint config, prettier config, eslint/prettier devDependencies, or format script existed at all, despite CONTRIBUTING.md stating "ESLint and Prettier are configured in the frontend/ directory" and instructing contributors to run npm run format before committing. Added eslint.config.mjs (flat config extending next/core-web-vitals + next/typescript + prettier, the standard create-next-app boilerplate), .prettierrc/.prettierignore matching this codebase's existing single-quote/semicolon style, the necessary devDependencies, and format/format:check scripts.
• [Frontend Scaffolding] Recreate typed environment loader and .env.example #1307: src/lib/env.ts, frontend/.env.example, and src/lib/__tests__/env.test.ts already exist in full — nothing was deleted, and .env.example already documents both NEXT_PUBLIC_API_URL and NEXT_PUBLIC_TTS_API_URL (added for Expand Dispute Window for Global Participation #116's TTS integration). The real gap: env.test.ts had zero test coverage of NEXT_PUBLIC_TTS_API_URL's validation branch despite env.ts already implementing it. Added the missing valid/unset/empty-string/invalid-URL cases.
• [Frontend Scaffolding] Configure Jest + Testing Library #1308: jest.config.js and jest.setup.js already exist in full — jest-environment-jsdom, @testing-library/react, @testing-library/jest-dom, and jest-axe are all wired up together with coverage thresholds enforced via test:ci, exactly as described. No gap found here; verified against package.json's devDependencies and jest.setup.js directly.
• [Frontend Scaffolding] Recreate package.json scripts expected by CI #1306: package.json's script surface (test, test:ci, every test:e2e:* variant, test:a11y, lighthouse, axe, bundlewatch, generate-client, analyze) already matches every script name invoked by name across .github/workflows/*.yml. The one real gap found on closer inspection: a11y:manual pointed at scripts/manual-testing-checklist.js, which did not exist in this repo (its siblings axe-audit.js, lighthouse-audit.js, and run-e2e-tests.js all exist; this one didn't) — running it failed with "Cannot find module". Added the missing script.

Test plan

• Verified brace/paren balance and that package.json still parses as valid JSON via static analysis (no build/test/install commands run)
• Traced every script name in .github/workflows/*.yml (test.yml, accessibility.yml, dependency-scan.yml, license-check.yml, e2e-staging.yml) against package.json's script surface to confirm [Frontend Scaffolding] Recreate package.json scripts expected by CI #1306's true (and only) gap
• Confirmed .license-checker.json, scripts/axe-audit.js, scripts/lighthouse-audit.js, and scripts/run-e2e-tests.js all already exist before ruling out further [Frontend Scaffolding] Recreate package.json scripts expected by CI #1306 gaps

Closes #1305
Closes #1306
Closes #1307
Closes #1308

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