2156 feature implement invitation flow for users without an account

[KarinePistili]

Description

This PR implements a token-based invitation flow for users who do not yet have an account, providing a smoother onboarding experience while reusing the existing invitation acceptance flow.

Changes

• Added support for secure invitation tokens stored in Firestore.
• Implemented the new /invites/:token flow.
• Added invitation validation before allowing access.
• Preserved the invitation token during the sign-in/sign-up process and automatically resumed the flow after authentication.
• Added support for both private (email-restricted) and public invitations.
• Added a pending invitation dialog displayed after authentication when applicable.
• Stored deferred invitations as user notifications when the user chooses to accept them later.
• Added invitation metadata (studyTitle, isPublic, etc.) to improve the user experience.
• Updated the invitation UI and localization strings.

Result

• Invited users can create an account and continue the invitation flow without losing context.
• Invitations are validated before acceptance.
• Users who postpone accepting an invitation can access it later through their notifications.
• The implementation prepares the invitation system for future link-based invitation improvements.

Issue #2156

[github-actions]

⚠️ PR Description Issues Detected

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

Please update the PR description to address these issues.

[github-actions]

🚫 Too many open Pull Requests

You already have 3 open PRs in this repository.

Please finish or close existing PRs before opening new ones.

[sonarqubecloud]

Quality Gate passed

Issues
8 New issues
0 Accepted issues

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

See analysis details on SonarQube Cloud