Migrate from App Engine to Firebase Hosting

[max-ostapenko]

Summary

Part 4 of splitting #1276 into sequential PRs (based on #1332 merge-har-docs).

This PR completes the migration from Google App Engine to Firebase Hosting:

• Adds .firebaserc configuring the httparchive default project
• Adds firebase.json configured for static hosting from dist, clean URLs, security headers, immutable caching for fonts, and local emulator on port 8080
• Removes app.yaml
• Updates .github/workflows/deploy.yml to deploy to Firebase Hosting (live channel) using FirebaseExtended/action-hosting-deploy
• Adds .github/workflows/firebase-preview.yml to deploy preview channels for pull requests
• Updates package.json scripts (start, stage, deploy) for Firebase Hosting
• Updates tools/scripts/run_and_test_website.sh and run_and_test_website.bat to test against the Firebase Hosting emulator

Sequence

1. Migrate from Python to JS (staying on AppEngine) #1311 (migrate-astro-appengine into main)
2. Lazy load CWV distribution histogram on user click #1328 (courageous-galliform into migrate-astro-appengine)
3. Merge har.fyi under docs #1332 (merge-har-docs into courageous-galliform)
4. This PR (migrate-firebase-hosting into merge-har-docs)

[github-actions]

Visit the preview URL for this PR (updated for commit 7fa9785):

https://httparchive--pr1333-migrate-firebase-hos-o2egoiz2.web.app

_{(expires Mon, 21 Sep 2026 20:31:49 GMT)}

_{🔥 via Firebase Hosting GitHub Action 🌎}

_{Sign: 1069cba7974889c25bacf4f0a19ddb6735592729}