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.

CMakeLists.txt

@@ -63,6 +63,7 @@ add_executable(udpscope
   src/plot_host.cpp
   src/export.cpp
   src/layout_store.cpp
+  src/source_dialog.cpp
   src/measure.cpp
   src/math_channel.cpp
   src/ui.cpp