Feat: Remove user list dependency from invitation flow

[KarinePistili]

Summary

This PR replaces the user selection dropdown with an email-based invitation flow.

Changes

• Removed the user dropdown that loaded all registered users.
• Added an email input for inviting participants.
• Updated the invitation flow to accept an email address instead of selecting an existing user.
• If the invited email belongs to an existing account, an in-app invitation is created for that user.
• If the invited email does not belong to an existing account, an invitation email is sent to encourage account creation (the complete invitation flow for new users will be implemented in a follow-up issue).

Benefits

• Eliminates the need to read the entire users collection from Firestore.
• Improves user privacy by no longer exposing registered users' email addresses.
• Reduces Firestore read operations.
• Provides a simpler and more scalable invitation workflow.

Future Work

A follow-up implementation will introduce token-based invitation links for users without an account, allowing them to register and automatically resume the invitation flow. Issue: #2156

Closes issues: #2149 and #2150

[github-actions]

⚠️ PR Description Issues Detected

• Missing issue reference (e.g., 'Fixes Graphics #123')

Please update the PR description to address these issues.

[sonarqubecloud]

Quality Gate passed

Issues
3 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.5% Duplication on New Code

See analysis details on SonarQube Cloud