refactor(typesetting): 替换html2canvas为SnapDOM v2.12.0 - 解决渲染不一致问题

- 新增 public/static/lib/snapdom/snapdom.js (IIFE构建)
- phone_image.html script引用从html2canvas改为snapdom
- phone-image.js 3处截图调用全部替换为snapdom.toCanvas()
  - capturePageViaHtml2Canvas (分页截图)
  - 长图生成 (scale:2)
  - 导出单页 (scale:2)
- API映射: html2canvas(elem,{scale,useCORS,width,height,logging}) -> snapdom.toCanvas(elem,{scale,backgroundColor})
- 返回值兼容: Promise<Canvas> 下游toDataURL不受影响
- 保留html2canvas目录不删除(output.html仍引用)

view/admin/post/phone_image.html

@@ -279,7 +279,7 @@
     <script src="/static/lib/jquery/jquery-3.4.1.min.js"></script>
     <script src="/static/lib/layui/layui.js"></script>
     <script src="/static/lib/wangeditor/index.js"></script>
-    <script src="/static/lib/html2canvas/html2canvas.js"></script>
+    <script src="/static/lib/snapdom/snapdom.js"></script>
     <script src="/static/js/phone-image.js"></script>
     <script>
         layui.use(['form', 'layer'], function () {