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

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

Services/Exports/MonthlyTimesheetExcelExportService.cs

@@ -0,0 +1,33 @@
+using WorkTracker.Services.WorkDays;
+
+namespace WorkTracker.Services.Exports;
+
+public sealed class MonthlyTimesheetExcelExportService(
+    IWorkDayService workDayService,
+    IMonthlyTimesheetExcelExporter exporter,
+    IWebHostEnvironment environment) : IMonthlyTimesheetExcelExportService
+{
+    private readonly IWorkDayService workDayService = workDayService;
+    private readonly IMonthlyTimesheetExcelExporter exporter = exporter;
+    private readonly IWebHostEnvironment environment = environment;
+
+    public async Task<MonthlyTimesheetExcelFile> ExportAsync(int year, int month, bool includePreview, CancellationToken cancellationToken = default)
+    {
+        var templatePath = Path.Combine(environment.ContentRootPath, "Templates", "monthly-timesheet-template.xlsx");
+        if (!File.Exists(templatePath))
+        {
+            throw new FileNotFoundException("Monthly timesheet template not found.", templatePath);
+        }
+
+        var timesheet = await workDayService.GetMonthlyTimesheetAsync(year, month, includePreview, cancellationToken);
+
+        await using var templateStream = File.OpenRead(templatePath);
+        var content = exporter.Export(timesheet, templateStream);
+
+        return new MonthlyTimesheetExcelFile
+        {
+            FileName = $"timesheet-{year}-{month:00}.xlsx",
+            Content = content
+        };
+    }
+}