fix(pdf-generator): 使用自定义外观流嵌入签名图片

- 恢复使用 widget.setNormalAppearance() 方式
- 创建 XObject Form 作为外观流
- 签名图片按字段尺寸等比例缩放并居中
- 不依赖页面索引，直接设置到widget上

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/services/planting-service/src/infrastructure/pdf/pdf-generator.service.ts

@@ -383,41 +383,48 @@ export class PdfGeneratorService {
           const form = pdfDoc.getForm();
           const signatureButton = form.getButton(FORM_FIELDS.SIGNATURE);
 
-          // 获取按钮的 widget 和位置
+          // 获取按钮的 widget 和尺寸
           const widgets = signatureButton.acroField.getWidgets();
           if (widgets.length > 0) {
             const widget = widgets[0];
             const rect = widget.getRectangle();
+            const { width: fieldWidth, height: fieldHeight } = rect;
 
-            // 获取签名图片原始尺寸
+            // 计算图片缩放尺寸（保持宽高比，适应字段大小）
             const imgDims = signatureImage.scale(1);
-
-            // 按字段尺寸等比例缩放签名图片
-            // 计算宽度和高度的缩放比例，取较小值以确保签名完全在字段内
-            const scaleX = rect.width / imgDims.width;
-            const scaleY = rect.height / imgDims.height;
-            const scale = Math.min(scaleX, scaleY);
-
+            const scale = Math.min(fieldWidth / imgDims.width, fieldHeight / imgDims.height);
             const scaledWidth = imgDims.width * scale;
             const scaledHeight = imgDims.height * scale;
 
-            // 在字段内居中放置签名
-            const offsetX = (rect.width - scaledWidth) / 2;
-            const offsetY = (rect.height - scaledHeight) / 2;
+            // 计算居中位置
+            const x = (fieldWidth - scaledWidth) / 2;
+            const y = (fieldHeight - scaledHeight) / 2;
 
-            // 获取按钮所在的页面
-            const pages = pdfDoc.getPages();
-            const page = pages[5]; // 第6页（签名区域）
+            // 创建外观流内容
+            const appearanceStream = `q ${scaledWidth} 0 0 ${scaledHeight} ${x} ${y} cm /Img Do Q`;
 
-            // 在按钮位置绘制缩放后的签名图片（居中）
-            page.drawImage(signatureImage, {
-              x: rect.x + offsetX,
-              y: rect.y + offsetY,
-              width: scaledWidth,
-              height: scaledHeight,
+            // 创建 XObject Form 作为外观
+            const context = pdfDoc.context;
+            const imageXObjectName = 'Img';
+
+            const Resources = context.obj({
+              XObject: { [imageXObjectName]: signatureImage.ref },
             });
 
-            this.logger.log(`Signature embedded at (${rect.x + offsetX}, ${rect.y + offsetY}) with size ${scaledWidth}x${scaledHeight}, field size: ${rect.width}x${rect.height}`);
+            const appearanceStreamRef = context.register(
+              context.stream(appearanceStream, {
+                Type: 'XObject',
+                Subtype: 'Form',
+                FormType: 1,
+                BBox: [0, 0, fieldWidth, fieldHeight],
+                Resources,
+              }),
+            );
+
+            // 设置外观到 widget
+            widget.setNormalAppearance(appearanceStreamRef);
+
+            this.logger.log(`Signature embedded using custom appearance stream, field size: ${fieldWidth}x${fieldHeight}`);
           } else {
             throw new Error('No widgets found for signature button');
           }