Implement feature user block

[alupcosta]

Description

Implements the User block of the domain layer: Voter and Administrator, plus two context-specific administrator subclasses (CondominiumAdministrator, OnuAdministrator) that store data relevant to vote weighting.

Type of Change

• New feature (non-breaking change which adds functionality)

Changes Made

• Voter: id and name, validated on creation
• Administrator (base): registers voters with duplicate-id protection; controls voting open/close state
• CondominiumAdministrator: registers a condo member and stores their ideal fraction
• OnuAdministrator: registers a member and stores permanent member status
• Unit tests for all classes above (16 tests), 100% coverage on this block's files

How to Test

1. npm install
2. npm run lint
3. npm run test:coverage

Related Issues

Closes #4

Checklist

• Self-reviewed my code
• Added tests that prove the fix is effective or that the feature works
• Existing local tests pass successfully