Replace double with deterministic money representation

[v1dit]

Summary

CodeRabbit identified potential precision issues caused by using double for monetary values in financial calculations.

Problem

Financial values should use a deterministic representation to avoid floating-point rounding errors.

Affected areas:

• Split.amount
• Expense.amount
• Settlement.amount
• calculation_utils.dart
• settlement.dart
• validation_utils.dart

Proposed Solution

• Replace money values with integer cents representation
• Update validation logic
• Update settlement calculations
• Update unit tests

Reference

Related to PR #40