Restore and verify automatic GitHub Pages deployment from main

[beanbean9339]

Context

Follow-up to #40. OpenCite is currently served from the legacy gh-pages branch (Settings > Pages reports build_type: legacy). There is no automatic Pages deploy running from main yet. PR #72 introduces an official GitHub Pages artifact/deploy workflow, but local tests and PR CI alone do not verify that the live site updates.

Work

• Merge Deploy current main to GitHub Pages automatically #72 after review. Keep the existing gh-pages branch as a fallback until deployment is verified.
• Change Settings > Pages > Build and deployment > Source to GitHub Actions. This is a one-time repository setting; no deployment token should be needed.
• If the merge-triggered workflow ran before the source setting changed, run Deploy GitHub Pages manually once from main.
• Verify that the deployment workflow succeeds and https://imageomics.github.io/OpenCite/ loads the newly built app and assets under /OpenCite/.
• Confirm a subsequent push to main triggers a successful deploy and updates the live site without manual steps.

Acceptance criteria

• Pages uses GitHub Actions as its deployment source.
• The Deploy GitHub Pages workflow completes successfully from main after tests, metadata validation, and build.
• The live site serves the built output, including the expected current commit's assets.
• A later push to main updates the live site automatically; no manual deployment is needed for future commits.

Related: #40 (original completed deployment task); implementation PR: #72. Keep this issue open until the live behavior is verified, not merely until the PR merges.