feat(ui): add PDF export via browser print

Adds a PDF button to the report header. Clicking it opens
/chart/current?print=true in a new tab, which auto-triggers
window.print() so the user can save to PDF via the browser dialog.

- chart submodule bumped: PrintMode support (no filter JS, auto-print,
  @media print CSS)
- handlers.go: passes PrintMode=true when ?print=true query param is set
- index.html: PDF button alongside Raw Data / Reanimator
- app.js: printReport() helper; button shown/hidden with other exports

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

web/static/js/app.js

@@ -1410,6 +1410,7 @@ async function loadData(vendor, filename) {
     document.getElementById('clear-btn').classList.remove('hidden');
     document.getElementById('header-raw-btn').classList.remove('hidden');
     document.getElementById('header-reanimator-btn').classList.remove('hidden');
+    document.getElementById('header-pdf-btn').classList.remove('hidden');
     document.getElementById('header-log-meta').classList.remove('hidden');
 
     loadAuditViewer();
@@ -1509,6 +1510,10 @@ function exportData(format) {
     window.location.href = `/api/export/${format}`;
 }
 
+function printReport() {
+    window.open('/chart/current?print=true', '_blank');
+}
+
 // Clear data
 async function clearData() {
     try {
@@ -1518,6 +1523,7 @@ async function clearData() {
         document.getElementById('clear-btn').classList.add('hidden');
         document.getElementById('header-raw-btn').classList.add('hidden');
         document.getElementById('header-reanimator-btn').classList.add('hidden');
+        document.getElementById('header-pdf-btn').classList.add('hidden');
         document.getElementById('header-log-meta').classList.add('hidden');
         document.getElementById('upload-status').textContent = '';
         const frame = document.getElementById('audit-viewer-frame');