docs: add a runnable Elements-to-editor workflow

[adeelraza]

Adds an optional Elements example to React Email Editor: generate a welcome email from React, edit it visually, persist the edited design, and export HTML or JSON. The README keeps the editor wrapper as its focus, with a short, self-contained Elements paragraph near the bottom.

Changes

• Correct the quick start’s design-JSON guidance, gate export on readiness, show exported HTML, and clarify React 18 versus classic React 16.8/17 rendering.
• Add a complete integration guide and lazy-loaded /elements demo. The demo saves/reopens edits locally, exposes the React source, and offers HTML/JSON inspection, copying, and downloading.
• Recover from a missing export callback after 30 seconds; ignore late callbacks after timeout, retry, or unmount. Discard malformed or unsupported saved demo data while safely handling blocked storage.
• Keep Context7 rules self-contained with a complete workflow and absolute guide URL. Validate against the official schema and keep each rule under 255 characters.
• Document the built-package development workflow: root npm start alongside npm --prefix demo run dev, using Node 22 from .nvmrc.
• Run demo checks on Node 22 and 25 in a separate path-filtered workflow. Main wrapper CI stays unfiltered, and demo CI relies on the existing prepare build during npm ci.
• Compile only complete TSX snippets marked with <!-- check-docs -->; allow multiple examples and report failures at their original Markdown lines. Use jsdom’s browser storage in tests to avoid the Node 25 native-storage conflict.

Validation

All commands below passed on both Node 22.23.3 and Node 25.9.0, after clean installs and without disabling Node’s storage API:

npm ci
npm run lint
npm run typecheck
npm test
npm --prefix demo ci
npm --prefix demo run build
npm --prefix demo test
npm --prefix demo run test:docs

Each runtime passed 12 wrapper tests and 10 demo tests, the production demo build, and both marked documentation examples. Additional checks passed for changed-demo formatting, Context7 schema validation, whitespace, and documentation-checker fixtures (multiple independent examples, ignored fragments, correct error locations, and missing-marker errors).

The demo tests use the real Elements renderer and a mocked hosted-editor boundary. Storage is browser-local; production applications need their own persistence. The narrow as EmailDesign assertion bridges Elements 0.1.22’s current declarations and uses the same explanation in both examples. Wrapper source and published-package configuration are unchanged.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.43%. Comparing base (c5fc76b) to head (dacf25c).

Additional details and impacted files

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  Coverage   97.43%   97.43%           
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  Files           2        2           
  Lines          78       78           
  Branches       20       20           
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  Hits           76       76           
  Partials        2        2           

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