Support modern bundler imports and prepare v3.2.4

[infeng]

Summary

• export the Viewer component directly from the UMD package entry so documented default imports work in modern bundlers
• update the Next.js client-only example to unwrap the CommonJS default explicitly
• add a release verification check for the built package export
• bump the patch version to 3.2.4

Related to #198. This does not add server-side Viewer rendering; the documented client-only Next.js integration remains required.

Reproduction

The 3.2.3 tarball built successfully with current tools, but default imports resolved to a module object at runtime. React 19/Vite 8 reported an invalid element type, and Next.js 16 reported React error 306.

Validation

• npm run verify
• public npm registry scan
• React 19.2.8 + Vite 8.2.2 production build
• Next.js 16.3.1 + React 19.2.8 production build with a statically prerendered App Router page
• Chromium 149.0.7827.55: both fixtures hydrated, opened Viewer, loaded an image, and closed with Escape without page errors
• npm pack dry run for react-viewer@3.2.4