Replace UTF-8 currency symbol with ASCII in Statistics visualization

[Copilot]

Statistics chart visualization was displaying garbled characters (乱码) in terminals without proper UTF-8 encoding support due to use of the Chinese Yuan symbol ¥ (U+00A5).

Changes

• src/Visualization.cpp: Replaced ¥ with $ in all currency displays (8 instances across 3 functions)
  • displayTimeBarChart(): income, expense, balance output
  • displayCategoryPieChart(): amount and total output
  • displayAccountBarChart(): income, expense, balance output

This aligns currency symbols across the codebase (main.cpp already uses $) and ensures consistent rendering in all terminal environments.

// Before
std::cout << " ¥" << std::fixed << std::setprecision(2) << stat.income << std::endl;

// After
std::cout << " $" << std::fixed << std::setprecision(2) << stat.income << std::endl;

Original prompt

在我实际测试时，发现Statistics绘制图表的时候出现乱码，请为我修复

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