feat(pdf): Add page numbering to generated PDF documents

- Add page counter display at bottom center of each PDF page
- Track total page count and current page index during PDF generation
- Use Arial font (10pt) with gray color (#999999) for page numbers
- Format page numbers as "current / total" (e.g., "1 / 5")
- Position page numbers 20pt from bottom of page
- Refactor loop from foreach to indexed for loop to access page index
- Improve PDF document readability with page navigation information

server/MiAssessment/src/MiAssessment.Core/Services/PdfGenerationService.cs

@@ -409,17 +409,27 @@ public class PdfGenerationService : IPdfGenerationService
     private static void BuildAndSavePdf(List<byte[]> images, string filePath)
     {
         using var document = new PdfDocument();
+        var totalPages = images.Count;
+        var font = new XFont("Arial", 10, XFontStyleEx.Regular);
+        var brush = new XSolidBrush(XColor.FromArgb(153, 153, 153)); // #999999
 
-        foreach (var imageBytes in images)
+        for (var i = 0; i < totalPages; i++)
         {
+            var imageBytes = images[i];
             var page = document.AddPage();
             page.Width = XUnit.FromPoint(PageWidthPt);
             page.Height = XUnit.FromPoint(PageHeightPt);
 
-            using var stream = new MemoryStream(imageBytes);
             using var xImage = XImage.FromStream(() => new MemoryStream(imageBytes));
             using var gfx = XGraphics.FromPdfPage(page);
             gfx.DrawImage(xImage, 0, 0, page.Width, page.Height);
+
+            // 绘制页码（底部居中）
+            var pageNumber = $"{i + 1} / {totalPages}";
+            var size = gfx.MeasureString(pageNumber, font);
+            var x = (page.Width - size.Width) / 2;
+            var y = page.Height - 20; // 距底部 20pt
+            gfx.DrawString(pageNumber, font, brush, x, y);
         }
 
         document.Save(filePath);