feat(examples): add runnable Next.js App Router example (#21)

[rajarshidattapy]

Summary

Adds a runnable Next.js 15 App Router example under examples/nextjs-app-router/ that renders an Elements email with renderToHtml() in a Server Component and previews it at /email-preview. Closes #21.

Changes

• examples/nextjs-app-router/ — new example:
  • src/emails/welcome-email.tsx — a realistic transactional email as a props-driven component tree
  • src/app/email-preview/page.tsx — Server Component calling renderToHtml() + renderToPlainText(), previewing the result in an iframe
  • src/app/email-preview/raw/route.tsx — Route Handler serving the rendered HTML as text/html, the shape production code usually takes
  • README.md with setup instructions and the Server Component caveat
• README.md (root) — new Examples section linking the example
• pnpm-workspace.yaml — excluded the example; it has its own pnpm-workspace.yaml so pnpm install inside the directory installs the example alone (see Notes)

Test Plan

• pnpm build passes
• pnpm test passes (450 tests, 36 files)
• Tested in Storybook — N/A, no library or UI-component change
• Added/updated tests for new behavior — see Notes

Manually verified end to end:

cd examples/nextjs-app-router && pnpm install && pnpm dev

/ → 200, /email-preview → 200, /email-preview/raw → 200 text/html containing 33 tables and 54 Outlook conditional comments. Preview page checked in headless Chromium: email renders in the frame, no console or page errors. pnpm build in the example passes. Root pnpm install && pnpm build unaffected.

Notes

Why the example installs from npm instead of workspace:*. My first version was a workspace member. Both pnpm dev and next build --turbopack failed:

x You're importing a component that needs `createContext` …
x You're importing a component that imports react-dom/server …
  at ../../packages/react/dist/index.js

pnpm symlinks the workspace copy, Next resolves it to a path outside node_modules, compiles it as first-party source, and its RSC checks reject the barrel. serverExternalPackages: ["@unlayer/react-elements"] does not fix it — I tried; that list is matched against node_modules resolutions, which a symlinked workspace package no longer is.

Installing @unlayer/react-elements@^0.1.20 from npm lets Next externalise it on the server like any other dependency, and matches what a real consumer does. next.config.ts is empty as a result. The two workspace-config changes exist only to keep pnpm from re-linking the local copy.