Simplify embedded editor installation flow

[erseco]

Summary

This PR simplifies the embedded editor installation flow in the Omeka S module and aligns it more closely with the server-driven approach used in mod_exeweb.

What changed

• replaced the adaptive browser/proxy/jsDelivr install path with a server-only installation flow
• switched latest-version discovery from the GitHub API to the GitHub releases Atom feed
• removed chunked upload, browser ZIP download, proxy fallback, and manual ZIP install code paths
• added persisted installation phases so the admin UI can poll for status updates
• updated the installer UI to refresh its state in place without location.reload(), which avoids breaking Playground views
• added status endpoints for both the admin controller and API controller
• updated the PR Playground preview workflow so it:
  • uses the PR head owner/repo/ref when building the module ZIP URL
  • does not run for external fork PRs

Notes

• runtime editor loading remains local-only and still uses dist/static
• this change does not introduce a separate admin-installed source location yet
• the README now reflects the server-side installation model

Validation

• php -l passes for the updated PHP files
• legacy references to installFromFile, jsDelivr, chunk upload, and proxy fallback were removed from the main installer flow

Follow-up

The remaining useful validation is a manual check in Omeka S / Playground to confirm:

• the editor installs without reloading the page
• the status block changes from not installed to installed
• long-running installs recover correctly through status polling
• the PR preview comment uses the branch ZIP for same-repo PRs only

[github-actions]

omeka-s Playground Preview

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⚠️ The embedded eXeLearning editor is not included in this preview. You can install it from Modules > ExeLearning > Configure using the "Download & Install Editor" button. All other module features (ELPX upload, viewer, preview) work normally.