feat(preview): support loading the Preview library from npm

[jackiejou]

Summary

Adds an opt-in path for loading the Preview library from the box-content-preview npm package instead of injecting CDN script/stylesheet tags, gated on the useNpmBoxContentPreview feature flag.

• Flag off (default): behavior is unchanged; preview.js and preview.css are loaded from the CDN and global.Box.Preview is used.
• Flag on: box-content-preview and its styles.css are loaded via dynamic import, and the named Preview export is used. Two option groups are forwarded to Preview.show() that the npm build cannot self-derive from a CDN script tag:
  • pdfjs: { workerSrc } from the new optional pdfjsWorkerSrc prop, so document viewers can boot the pdfjs worker the consumer bundled
  • location: { staticBaseURI, version, locale } derived from the existing staticHost/staticPath/previewLibraryVersion/language props, so viewers that build asset URLs from staticBaseURI keep working

Requires box-content-preview >= 3.62.0 (box/box-content-preview#1704), declared as an optional peer dependency; consumers on the CDN path do not need to install it. Consumers on the npm path must also bundle a pdfjs-dist worker matching the version pinned by their installed box-content-preview.

Details

• A failed dynamic import (or a module missing the Preview export) surfaces through the standard error path: loading state ends, error state renders, and the host onError callback fires with the underlying cause.
• The flag is evaluated at mount; changing it after mount has no effect.
• box-content-preview@3.62.1 is added as a devDependency so Flow, jest, and the webpack builds resolve the real package. Two build-config accommodations exist only because this repo is itself the box-ui-elements peer that box-content-preview imports: a box-ui-elements/es -> src/ alias in the webpack/Storybook configs (the published es/ output does not exist in the working tree), and a babel-loader exclude for box-content-preview's pre-bundled dist (it ships modern syntax and needs no transpilation). Neither affects consumers.

Testing

• New unit tests cover: flag off (CDN injection unchanged), flag on (dynamic import, no tag injection, show() option shapes including staticHost trailing-slash handling), pdfjs omitted when pdfjsWorkerSrc is absent, and both import-failure paths.
• yarn flow check, eslint, and the full ContentPreview test suite (158 tests) pass. The production webpack build and Storybook build compile with the npm package resolved.