decap-cms-app README script-tag install is broken for React 19 — DCMS-001

[sempostma]

Promise (packages/decap-cms-app/README.md): the "Install via script tag" section tells consumers to load React from unpkg UMD builds matching the package's own peer dependency range (^19.1.0, see packages/decap-cms-app/package.json):

<script src="https://unpkg.com/react@^19/umd/react.production.min.js"></script>
<script src="https://unpkg.com/react-dom@^19/umd/react-dom.production.min.js"></script>

Reality: React 19 dropped UMD builds entirely — react@19.1.0's published tarball contains no umd/ directory (only cjs/, jsx-runtime.js, etc.), so both unpkg URLs 404. This isn't optional either: scripts/webpack.js:120-124,151-153 externalizes every peerDependencies key to a global name for the umd build target, so decap-cms-app's UMD bundle genuinely expects window.React/window.ReactDOM to exist — there's no working way to follow the documented script-tag instructions for the version the package itself requires.

This is a more specific, root-cause finding for the symptom already reported in #7878 (webpack/peer-dep warnings when bundling decap-cms-app with React 19) — that issue is about the bundler path; this one is about the README's own standalone script-tag example being dead on arrival for the exact peer version it names.

Acceptance:

• Pinning test/fixture (e.g. an HTML fixture or a CI check) asserting the react/react-dom URLs referenced in decap-cms-app's README resolve to real UMD assets for the currently pinned peer dependency range, so a future peer-dep bump can't silently reintroduce a dead example.
• typecheck/lint/test:unit green.