Add font support for chart legends

[Copilot]

This PR implements font support for chart legends, allowing users to specify font properties such as family, size, color, bold, italic, underline, and strike for legend text.

Changes Made

Core Implementation

• xmlChart.go: Added Font field to ChartLegend struct
• drawing.go:
  • Initialize TxPr (text properties) when creating chart legends
  • Apply font settings using the existing drawChartFont function
• chart.go: Updated documentation to include the new Font option

Testing

• chart_test.go: Added comprehensive TestChartLegendFont test covering:
  • Normal legend with font settings
  • Legend with position: "none" (edge case)
  • Different font properties validation

Usage Example

err := f.AddChart("Sheet1", "E1", &excelize.Chart{
    Type: excelize.Col,
    Series: series,
    Title: []excelize.RichTextRun{{Text: "Chart Title"}},
    Legend: excelize.ChartLegend{
        Position: "bottom", 
        Font: excelize.Font{
            Family: "Aptos", 
            Color:  "#3E3E3E", 
            Size:   10,
            Bold:   true,
        },
    },
})

Implementation Details

The implementation follows the same pattern used for other chart text elements (data labels, axes) to ensure consistency:

1. Text Properties Initialization: Legends now have proper TxPr structure with body properties and paragraph settings
2. Font Application: Uses the existing drawChartFont function to apply font settings to the legend's text properties
3. Edge Case Handling: Properly handles cases where legend position is "none" without errors

Testing Results

• All existing tests pass
• New test validates font functionality
• Manual testing confirms font settings are applied correctly
• Edge cases (no legend) handled properly

The changes are minimal and surgical, adding exactly the functionality requested without affecting existing behavior.

Fixes #2169.

---

💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.