fix(MediaSettings): track actual guest username

[Antreesy]

☑️ Resolves

• Follow-up to fix: reduce amount of requests to server during high guests activity #16361
• MediaSettings should rely on the same value as in the store

🖌️ UI Checklist

🖼️ Screenshots / Screencasts

🏚️ Before	🏡 After

🏁 Checklist

• 🌏 Tested with different browsers / clients:
  • Chromium (Chrome / Edge / Opera / Brave)
  • Firefox
  • Safari
  • Talk Desktop
  • Integrations with Files sidebar and other apps
  • Not risky to browser differences / client
• 🖌️ Design was reviewed, approved or inspired by the design team
• ⛑️ Tests are included or not possible
• 📗 User documentation in https://github.com/nextcloud/documentation/tree/master/user_manual/talk has been updated or is not required

[Antreesy]

/backport to stable32

[DorraJaouad]

Need to clean emit('update', newValue) from the other component
import { getGuestNickname } from '@nextcloud/auth' is not used