Registration with an already existing email succeeds without error

Description
When a user attempts to register in the mobile app using an email address that is already registered, the registration flow completes successfully without any error. The user can proceed through the full onboarding: register → create workspace → create project. Only when attempting to upload a project does the user see an error: "Failed to push changes. Ask the workspace owner to log in to their Mergin Maps dashboard for more information. (TrialExpired)".

The root cause is likely that duplicate email registration is not validated in the mobile app (or the server error is not surfaced), leaving the user in a broken account state.

Environment
[x] Staging [ ] PROD

Application (+ app version, build, operating system)
Mergin Maps iOS — 2026.4.0
iOS: 26.0

Steps to reproduce

1. Take an email address that already has a registered Mergin Maps account
2. Open the mobile app and tap Sign up
3. Fill in the registration form using the existing email
4. Complete the full onboarding flow (create workspace, create project)
5. Attempt to upload/sync the project

Actual results
Registration succeeds with no error. The user completes onboarding and creates a project.

Expected results
Registration with an already existing email should fail immediately with a clear error message (e.g. "An account with this email already exists"), preventing the user from entering a broken state.

Screenshots or logs

Conclusion
Created by Claude and supervised by a human test analyst.

Start at the mobile app's Sign up registration flow and reproduce the duplicate-email case on staging, then trace whether the server response is validated or surfaced. Done means an existing email stops registration immediately with a clear error, without allowing onboarding or project creation to continue.