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/templates/index.html

@@ -18,6 +18,7 @@
                 <button id="clear-btn" class="header-action hidden" onclick="clearData()">Clear Data</button>
                 <button id="header-raw-btn" class="header-action hidden" onclick="exportData('json')">Raw Data</button>
                 <button id="header-reanimator-btn" class="header-action hidden" onclick="exportData('reanimator')">Reanimator</button>
+                <button id="header-pdf-btn" class="header-action hidden" onclick="printReport()">PDF</button>
                 <button id="restart-btn" class="header-action" onclick="restartApp()">Restart</button>
                 <button id="exit-btn" class="header-action" onclick="exitApp()">Exit</button>
             </div>