fix(Pinia) - move guestNameStore initialisation from mixin to components

[Antreesy]

☑️ Resolves

• Possibly a regression from (guestNameStore) - Migrate to Pinia in guestNameStore #10475
• Fix cases, then there are guests in the participants list. Mixins aren't configured to work with Pinia stores, so they should be initialised at the component level

🚧 Tasks

• Follow-up: replace getParticipants.js mixin with composable

🏁 Checklist

• ⛑️ Tests (unit and/or integration) are included or not required
• 📘 API documentation in docs/ has been updated or is not required
• 📗 User documentation in https://github.com/nextcloud/documentation/tree/master/user_manual/talk has been updated or is not required
• 🔖 Capability is added or not needed