feat: add export-to-PDF via browser print with @media print CSS

webapp/static/index.html

@@ -109,6 +109,11 @@
         <div id="report-content" hidden>
           <!-- 顶部元信息条 -->
           <div class="report-meta" id="report-meta"></div>
+          <div class="report-actions no-print">
+            <button class="btn btn-ghost btn-export-pdf" id="export-pdf-btn" onclick="Report.exportPdf()">
+              📄 导出 PDF
+            </button>
+          </div>
 
           <!-- ① 指标均值卡片 -->
           <div class="section-label">① 指标均值 OVERVIEW</div>