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

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

Services/Exports/IMonthlyTimesheetExcelExportService.cs

@@ -0,0 +1,15 @@
+namespace WorkTracker.Services.Exports;
+
+public interface IMonthlyTimesheetExcelExportService
+{
+    Task<MonthlyTimesheetExcelFile> ExportAsync(int year, int month, bool includePreview, CancellationToken cancellationToken = default);
+}
+
+public sealed class MonthlyTimesheetExcelFile
+{
+    public required string FileName { get; init; }
+
+    public required byte[] Content { get; init; }
+
+    public string ContentType => "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet";
+}