[Tech Debt] FE migration away from uuid_user + BE removal

[carlosPc1987]

This technical debt issue tracks the follow-up work required after applying Plan B in the backend.

The frontend currently depends on the uuid_user field in multiple components, services, and tests.
Since FE is not available during this sprint, the backend has temporarily kept the field and marked it as deprecated.

Frontend tasks:

• Remove all usages of uuid_user across components, services, interfaces, mocks, and tests.
• Replace it with the proper FE-side user identifier (session/auth context).
• Ensure challenge resolution logic still works after removing the field.

Backend tasks (only after FE migration is merged):

• Remove uuid_user from SubmissionDto.
• Remove mapping from SubmissionMapper.
• Update backend tests.
• Update Swagger/OpenAPI documentation.

Condition:
Remove uuid_user only after merging the FE migration.