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.

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.24%. Comparing base (f47b3df) to head (c4e063c).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2171   +/-   ##
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  Coverage   99.23%   99.24%           
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  Files          32       32           
  Lines       30383    30404   +21     
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+ Hits        30152    30173   +21     
  Misses        153      153           
  Partials       78       78           

Flag	Coverage Δ
unittests	99.24% <100.00%> (+<0.01%)	⬆️

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[xuri]

LGTM.