fix(security): escape single quotes in GraphStorytellerExporter (CWE-79)

The esc() method was missing single-quote escaping, allowing potential
XSS via crafted node IDs containing single quotes in attribute contexts.
Added ' escaping to match the complete HTML escaping used by
InteractiveHtmlExporter.escHtml() and ExportUtils.escapeXml().

Author: sauravbhattacharya001

Gvisual/src/gvisual/GraphStorytellerExporter.java

@@ -372,6 +372,6 @@ private double computeStdDev(Collection<Integer> values) {
     private static String esc(String s) {
         if (s == null) return "";
         return s.replace("&", "&amp;").replace("<", "&lt;").replace(">", "&gt;")
-                .replace("\"", "&quot;");
+                .replace("\"", "&quot;").replace("'", "&#39;");
     }
 }