Add GitHub Pages deploy

[Nebyt]

Summary

• Adds a GitHub Pages workflow that builds the CRA app and publishes build/ on push to main (or via manual run).
• Sets PUBLIC_URL from the Pages base path and uses it as the React Router basename so the project site works at /dapp-example.
• Leaves the existing S3/CloudFront release deploy on root-relative assets.
• Keeps Pages concurrency.cancel-in-progress: false so a newer main push cannot abort an in-flight deploy and wedge Pages.

Test plan

• In the repo: Settings → Pages → Source: GitHub Actions
• Merge this PR (or run the GitHub Pages workflow on main)
• Open https://emurgo.github.io/dapp-example/ and confirm the app loads (JS/CSS/WASM)
• Confirm wallet connect still works over HTTPS
• Confirm a release still deploys to S3/CloudFront as before

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Note

Low Risk
CI/deploy and routing basename only; existing S3/CloudFront release path is unchanged. Low risk beyond Pages config and SPA 404 fallback.

Overview
Adds GitHub Pages deploy on main (and manual dispatch) so the CRA app can be hosted at the project-site path.

The workflow builds with PUBLIC_URL from the Pages base path, writes a production .env, and prepares SPA files (.nojekyll, 404.html). BrowserRouter now uses that PUBLIC_URL as basename so routing works under /dapp-example without changing root-relative S3/CloudFront deploys.

Bumps package version to 2.5.0. Pages concurrency does not cancel in-flight deploys.

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