fix(fireshare): rebuild client on update to fix nginx 500

[michelroegl-brunner]

✍️ Description

Fireshare LXC updates redeploy the app and refresh the Python backend but did not rebuild the React client. Nginx continues to serve /opt/fireshare/app/client/build, which is missing or stale after CLEAN_INSTALL=1 fetch_and_deploy_gh_release, resulting in 500 Internal Server Error after update (reported in #14812).

This adds npm install and npm run build in update_script(), matching the install script and peer scripts such as flatnotes.

🔗 Related Issue

Fixes #14812

✅ Prerequisites (X in brackets)

• Self-review completed – Code follows project standards.
• Tested thoroughly – Changes work as expected.
• No security risks – No hardcoded secrets, unnecessary privilege escalations, or permission issues.

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🛠️ Type of Change (X in brackets)

• 🐞 Bug fix – Resolves an issue without breaking functionality.
• ✨ New feature – Adds new, non-breaking functionality.
• 💥 Breaking change – Alters existing functionality in a way that may require updates.
• 🆕 New script – A fully functional and tested script or script set.
• 🌍 Website update – Changes to script metadata (PocketBase/website data).
• 🔧 Refactoring / Code Cleanup – Improves readability or maintainability without changing functionality.
• 📝 Documentation update – Changes to README, AppName.md, CONTRIBUTING.md, or other docs.

[geolup2000]

Thank you so much!