QA: Implement Unit Tests for core services

[omatheusmesmo]

Description

Current .spec.ts files are just scaffolds. We need real unit tests for critical business logic, especially in services.

Proposed Changes

• Implement unit tests for AuthService.
• Implement unit tests for ShoppingListService.
• Use Jasmine/Karma or a modern alternative like Vitest/Jest if planned.

Acceptance Criteria

• Core services have at least 80% code coverage.
• Tests pass in the CI environment.

[omatheusmesmo]

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