fix(bug):修复转换PDF功能BUG，优化园长签名生成逻辑，让更多的文档支持园长签名功能

config/config.py

@@ -78,7 +78,7 @@ def load_config(config_filename="config.toml"):
                 base_dir, paths.get("output_folder", "output")
             ),
             "signature_image": get_resource_path(
-                os.path.join("data", paths.get("signature_image", ""))
+                os.path.join("data", paths.get("signature_image", "signature.png"))
             ),
             "class_name": class_info.get("class_name", "未命名班级"),
             "teachers": class_info.get("teachers", []),
@@ -118,7 +118,9 @@ def save_config(config_data, config_filename="config.toml"):
                 "image_folder": os.path.basename(config_data.get("image_folder", "")),
                 "fonts_dir": os.path.basename(config_data.get("fonts_dir", "fonts")),
                 "signature_image": get_resource_path(
-                    os.path.join("data", config_data.get("signature_image", ""))
+                    os.path.join(
+                        "data", config_data.get("signature_image", "signature.png")
+                    )
                 ),
             },
             "class_info": {