feat(患者签署): 接入签署上下文接口并补齐文书渲染与手写签名链路

patient-h5/src/styles/index.css

@@ -389,6 +389,22 @@ button {
   font-weight: 400;
 }
 
+/* 占位文案要压在画布正中间，所以画布外面必须有一层定位容器 */
+.signature-field {
+  position: relative;
+}
+
+.signature-placeholder {
+  position: absolute;
+  inset: 0;
+  display: grid;
+  color: #c9d8de;
+  font-size: 15px;
+  letter-spacing: 2px;
+  pointer-events: none;
+  place-items: center;
+}
+
 .signature-canvas {
   display: block;
   width: 100%;