ask which source to open when --server is absent

The chooser runs before ScopeModel exists, so declining costs no buffer and no
socket. UDPSCOPE_PROBE_SOURCES=1 prints what the discovery found and exits,
before gtk_init(), so the list is checkable with no display.

src/main.cpp

@@ -5,6 +5,7 @@
 #include "config.hpp"
 #include "model.hpp"
 #include "plot_host.hpp"
+#include "source_dialog.hpp"
 #include "ui.hpp"
 
 #include <cstdio>
@@ -19,6 +20,10 @@ int main(int argc, char** argv) {
     return 2;
   }
 
+  // No `--server` means the user has not said which source to open, so ask. Before the model:
+  // declining must cost no buffer and no socket, and nothing below has to cope with "no source".
+  if (!cfg.server_specified && !udpscope::choose_source(cfg)) return 0;
+
   udpscope::ScopeModel model(cfg);
   udpscope::RowSet rows;
   rows.set_row_count(cfg.plots);