security: fix XSS vulnerabilities in GanttChart and AuditPage

- Add escapeHtml utility function for HTML entity encoding
- Apply escapeHtml to GanttChart popup HTML template
- Apply escapeHtml to AuditPage PDF export HTML template

This prevents potential XSS attacks if task names, user names,
or other dynamic content contains malicious HTML/JavaScript.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/src/utils/escapeHtml.ts

@@ -0,0 +1,24 @@
+/**
+ * Escapes HTML special characters to prevent XSS attacks.
+ * Use this when inserting dynamic content into HTML strings.
+ */
+export function escapeHtml(unsafe: string | null | undefined): string {
+  if (unsafe == null) return ''
+  return String(unsafe)
+    .replace(/&/g, '&')
+    .replace(/</g, '&lt;')
+    .replace(/>/g, '&gt;')
+    .replace(/"/g, '&quot;')
+    .replace(/'/g, '&#039;')
+}
+
+/**
+ * Escapes HTML for use in HTML attributes.
+ * More restrictive than escapeHtml - also escapes backticks and equals.
+ */
+export function escapeAttr(unsafe: string | null | undefined): string {
+  if (unsafe == null) return ''
+  return escapeHtml(unsafe)
+    .replace(/`/g, '&#96;')
+    .replace(/=/g, '&#61;')
+}