fix(mobile-app): adjust signature image ratio to match PDF field

The signature image was 600x200 (3:1 ratio) but the PDF signature
field is 92x51 (1.8:1 ratio). This caused the signature to be scaled
down to only 60% of the field height, making it appear too small.

Changed signature image dimensions to 460x255 (~1.8:1) to better
match the PDF field proportions and maximize signature size.

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

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

frontend/mobile-app/lib/features/contract_signing/presentation/widgets/signature_pad.dart

@@ -162,8 +162,10 @@ class _SignaturePadState extends State<SignaturePad> {
   Future<Uint8List?> _renderSignatureImage() async {
     try {
       // 创建画布
-      const width = 600.0;
-      const height = 200.0;
+      // 尺寸比例需要匹配PDF签名字段比例（约92:51 ≈ 1.8:1）
+      // 使用较大尺寸保证清晰度
+      const width = 460.0;
+      const height = 255.0;
 
       final recorder = ui.PictureRecorder();
       final canvas = Canvas(recorder, Rect.fromLTWH(0, 0, width, height));