feat: implement Excel export functionality for monthly timesheets with template support

Co-authored-by: Copilot <copilot@github.com>

tests/WorkTracker.Tests/WorkbookAssert.cs

@@ -0,0 +1,90 @@
+using ClosedXML.Excel;
+using Xunit;
+
+namespace WorkTracker.Tests;
+
+internal static class WorkbookAssert
+{
+    public static void Equivalent(string expectedPath, byte[] actualContent)
+    {
+        using var expectedWorkbook = new XLWorkbook(expectedPath);
+        using var actualStream = new MemoryStream(actualContent);
+        using var actualWorkbook = new XLWorkbook(actualStream);
+
+        var expectedWorksheet = expectedWorkbook.Worksheet(1);
+        var actualWorksheet = actualWorkbook.Worksheet(1);
+
+        Assert.Equal(expectedWorksheet.Name, actualWorksheet.Name);
+        Assert.Equal(expectedWorksheet.RangeUsed()?.RangeAddress.ToString(), actualWorksheet.RangeUsed()?.RangeAddress.ToString());
+        Assert.Equal(expectedWorksheet.MergedRanges.Select(range => range.RangeAddress.ToString()), actualWorksheet.MergedRanges.Select(range => range.RangeAddress.ToString()));
+
+        var usedRange = expectedWorksheet.RangeUsed() ?? throw new InvalidOperationException("Expected workbook must have a used range.");
+        foreach (var column in Enumerable.Range(usedRange.RangeAddress.FirstAddress.ColumnNumber, usedRange.ColumnCount()))
+        {
+            Assert.Equal(Math.Round(expectedWorksheet.Column(column).Width, 5), Math.Round(actualWorksheet.Column(column).Width, 5));
+        }
+
+        foreach (var cell in usedRange.Cells())
+        {
+            var actualCell = actualWorksheet.Cell(cell.Address.RowNumber, cell.Address.ColumnNumber);
+            Assert.Equal(GetCellValue(cell), GetCellValue(actualCell));
+            Assert.Equal(cell.FormulaA1, actualCell.FormulaA1);
+
+            if (string.IsNullOrEmpty(cell.FormulaA1))
+            {
+                Assert.Equal(cell.DataType, actualCell.DataType);
+            }
+
+            var expectedStyle = DescribeStyle(cell.Style);
+            var actualStyle = DescribeStyle(actualCell.Style);
+            Assert.True(expectedStyle == actualStyle, $"Style mismatch at {cell.Address}: expected '{expectedStyle}' actual '{actualStyle}'");
+        }
+    }
+
+    private static string GetCellValue(IXLCell cell)
+    {
+        if (!string.IsNullOrEmpty(cell.FormulaA1))
+        {
+            return string.Empty;
+        }
+
+        return cell.Value.ToString();
+    }
+
+    private static string DescribeStyle(IXLStyle style)
+    {
+        return string.Join(
+            "|",
+            style.NumberFormat.NumberFormatId,
+            style.NumberFormat.Format,
+            style.Fill.PatternType,
+            DescribeColor(style.Fill.BackgroundColor),
+            DescribeColor(style.Fill.PatternColor),
+            style.Font.FontName,
+            style.Font.FontSize,
+            style.Font.Bold,
+            style.Alignment.Horizontal,
+            style.Alignment.Vertical,
+            style.Alignment.WrapText,
+            style.Border.LeftBorder,
+            DescribeColor(style.Border.LeftBorderColor),
+            style.Border.RightBorder,
+            DescribeColor(style.Border.RightBorderColor),
+            style.Border.TopBorder,
+            DescribeColor(style.Border.TopBorderColor),
+            style.Border.BottomBorder,
+            DescribeColor(style.Border.BottomBorderColor),
+            style.Protection.Locked);
+    }
+
+    private static string DescribeColor(XLColor color)
+    {
+        return color.ColorType switch
+        {
+            XLColorType.Color => $"rgb:{color.Color.ToArgb()}",
+            XLColorType.Indexed => $"indexed:{color.Indexed}",
+                XLColorType.Theme => $"theme:{color.ThemeColor}:{Math.Round(color.ThemeTint, 12)}",
+            _ => color.ColorType.ToString()
+        };
+    }
+}