docs: document local development workflow

[jamilahmadzai]

Summary

• Expands the README with a getting-started workflow for cloning, installing, configuring NEXT_PUBLIC_STABLEROUTE_API_BASE, running the dev server, and verifying a PR locally.
• Documents every package.json script, including test:watch, and adds the coverage command form with npm test -- --coverage.
• Clarifies the default http://localhost:3001 API base from src/lib/apiClient.ts and which frontend pages expect the backend there.

Closes #120

Bounty: GrantFox OSS campaign for issue #120

Validation

• npm ci completed using the lockfile; npm reported existing dependency deprecation/audit warnings.
• npm run lint passes.
• npm test -- src/lib/__tests__/apiClient.test.ts src/lib/__tests__/quote.test.ts --runInBand passes: 2 suites, 18 tests.
• README consistency check passes: documented scripts match package.json, and the env var/default API base/Node floor match repo files.
• git diff --check passes.
• README preview: reviewed the Markdown table, numbered steps, and fenced commands in the PR diff.

Baseline notes

• npm run build is currently blocked on main by src/app/quote/Client.tsx:41 (Cannot find name 'assetsDiffer'). This PR is documentation-only and does not touch that code path.
• Full npm test -- --runInBand is blocked by the same existing quote-page issue: src/app/quote/page.test.tsx fails when the page references assetsDiffer.