feat: enhance Excel export functionality with improved styling and download handling

tests/WorkTracker.Tests/MonthlyTimesheetExcelExporterTests.cs

@@ -42,6 +42,43 @@ public sealed class MonthlyTimesheetExcelExporterTests
         Assert.Equal("TOTALE", worksheet.Cell("AH1").GetString());
     }
 
+    [Fact]
+    public void Export_ForMonthWhereSecondDayIsWeekend_OnlyHighlightsWeekendAndHolidayDays()
+    {
+        var exporter = new MonthlyTimesheetExcelExporter();
+        var timesheet = CreateTimesheet(new DateOnly(2026, 5, 1), new HashSet<DateOnly>
+        {
+            new(2026, 5, 4)
+        });
+        var templatePath = GetTemplatePath();
+
+        using var templateStream = File.OpenRead(templatePath);
+        var workbookBytes = exporter.Export(timesheet, templateStream);
+
+        using var workbook = new XLWorkbook(new MemoryStream(workbookBytes));
+        var worksheet = workbook.Worksheet(1);
+
+        var headerRegularFill = GetFillSignature(worksheet.Cell("C1"));
+        var headerSaturdayFill = GetFillSignature(worksheet.Cell("D1"));
+        var headerSundayFill = GetFillSignature(worksheet.Cell("E1"));
+        var headerHolidayFill = GetFillSignature(worksheet.Cell("F1"));
+        var headerNextWeekdayFill = GetFillSignature(worksheet.Cell("G1"));
+
+        Assert.NotEqual(headerRegularFill, headerSaturdayFill);
+        Assert.NotEqual(headerRegularFill, headerSundayFill);
+        Assert.Equal(headerSundayFill, headerHolidayFill);
+        Assert.Equal(headerRegularFill, headerNextWeekdayFill);
+
+        var bodyRegularFill = GetFillSignature(worksheet.Cell("C4"));
+        var bodySaturdayFill = GetFillSignature(worksheet.Cell("D4"));
+        var bodyHolidayFill = GetFillSignature(worksheet.Cell("F4"));
+        var bodyNextWeekdayFill = GetFillSignature(worksheet.Cell("G4"));
+
+        Assert.NotEqual(bodyRegularFill, bodySaturdayFill);
+        Assert.Equal(bodySaturdayFill, bodyHolidayFill);
+        Assert.Equal(bodyRegularFill, bodyNextWeekdayFill);
+    }
+
     private static MonthlyTimesheetModel CreateTimesheet(DateOnly monthStart, ISet<DateOnly> holidays)
     {
         var lastDay = monthStart.AddMonths(1).AddDays(-1);
@@ -116,4 +153,26 @@ public sealed class MonthlyTimesheetExcelExporterTests
 
         throw new DirectoryNotFoundException("Unable to locate the WorkTracker repository root.");
     }
+
+    private static string GetFillSignature(IXLCell cell)
+    {
+        var fill = cell.Style.Fill;
+
+        return string.Join(
+            "|",
+            fill.PatternType,
+            DescribeColor(fill.BackgroundColor),
+            DescribeColor(fill.PatternColor));
+    }
+
+    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()
+        };
+    }
 }