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>

internal/server/handlers.go

@@ -84,7 +84,10 @@ func (s *Server) handleChartCurrent(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	html, err := chartviewer.RenderHTML(snapshotBytes, title)
+	printMode := r.URL.Query().Get("print") == "true"
+	html, err := chartviewer.RenderHTMLWithOptions(snapshotBytes, title, chartviewer.RenderOptions{
+		PrintMode: printMode,
+	})
 	if err != nil {
 		http.Error(w, "failed to render chart: "+err.Error(), http.StatusInternalServerError)
 		return