frontend: fix tables expand beyond parent div

pywebio/html/css/app.css

@@ -80,6 +80,10 @@
     margin-bottom: 10px;
 }
 
+.markdown-body table {
+    word-break: break-all; /*  prevent tables from expanding beyond their parent div */
+}
+
 .CodeMirror {
     font-size: 13px
 }