No SSR / Next.js App Router smoke test despite the documented `use client` guarantee

[sidgaikwad]

The README makes an explicit SSR promise:

The component works out of the box in React Server Components environments (e.g. Next.js App Router) — it ships with the 'use client' directive and touches the DOM only inside effects.

That claim is backed by a tsup banner:

https://github.com/unlayer/react-image-editor/blob/628b507/tsup.config.ts#L11-L14

…but nothing verifies it. The whole suite runs in jsdom (vitest.config.ts), so every test renders client-side. There is no test that:

1. the built dist/index.js and dist/index.mjs actually start with 'use client' — a banner is a build-config detail that a bundler or config change could silently drop, and it wouldn't fail any current check;
2. the component renders without throwing under react-dom/server's renderToString — i.e. that the render path really does stay off the DOM until effects run.

Both are cheap:

// dist smoke test
expect(readFileSync('dist/index.mjs', 'utf8').startsWith("'use client';")).toBe(true);

// SSR smoke test (node environment)
expect(() => renderToString(<ImageEditor image="x" />)).not.toThrow();

The second needs a // @vitest-environment node file (or a second Vitest project) since the current global environment is jsdom. The first needs to run after npm run build, so it fits as a post-build step in the CI checks job.

Given that Next.js App Router is called out by name as a supported target, it's worth having the regression net.