fix(web-app, admin): Angular 21+ modernizing for feeds & authentication

[ktsorensenWX]

Summary

• Continuing angular 21+ modernizing by enabling standalone for admin authentication & feeds
• Signals were introduced where needed
• admin.module.ts was updated to remove old imports.

Testing

• Tests using npm returned 1043 succeses

To-Do

• Adding comments
• Web UI testing

[jclark118]

Could you also update the button style icons/colors to match the other pages while you're in here? See the users page for an example. All the stuff in this PR looks good though