musrview: add "Save graphics" to the interactive (root7/web) viewer

root6 musrview could save the plot as png/pdf/svg/...; the ROOT7 web
rewrite dropped that. Add a "Save graphics" toolbar entry: the server
renders the classic layout (same code path as --png/--pdf/... batch
export) into the current working directory and hands it to the browser
as a download, so the browser's own save dialog/download folder decides
where it lands -- a native OS folder picker isn't reachable from here,
since the --embedded window is a Qt6 QWebEngineView owned by ROOT itself,
which auto-accepts downloads with no dialog.

Also fixes rasterized batch export (png/gif/jpg) coming out blank when
triggered from an already-running interactive session: TCanvas decides
at *construction* time, from gROOT->IsBatch(), whether to build a real
GUI canvas or a headless one, so canvas.SetBatch(kTRUE) after the fact
was too late. Vector formats (pdf/eps/svg) don't need pixel readback and
were unaffected, which is why this went unnoticed. Fixed by toggling
gROOT's batch flag for the export canvas's lifetime.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017cuZPrSNaKjG8xHPVNwV16

src/musrview.cpp

@@ -446,7 +446,7 @@ int main(int argc, char *argv[])
 
   if (graphicsOutput) {
     for (auto &canvas : canvasList) {
-      if (!canvas->SaveGraphics(fileName, graphicsExtension)) {
+      if (!canvas->SaveGraphics(graphicsExtension)) {
         std::cerr << std::endl << ">> musrview **ERROR** " << canvas->GetLastError() << std::endl;
         result = PMUSR_MSR_RUN_ERROR;
       }