Reset device selection on agent swap or unload (#124)

set_live_printer_agent centralizes
the swap: deselect the machine,
clear stale sidebar state and the
previous agent's Other Devices, then
install the new agent (or null when
its provider vanished). Plugin
load/unload callbacks refresh the
dropdown and re-run agent selection.
load_last_machine no longer falls
back to the first available machine.

src/slic3r/GUI/Tab.cpp

@@ -7907,6 +7907,24 @@ bool TabPrinter::apply_extruder_cnt_from_cache()
     return false;
 }
 
+void TabPrinter::refresh_printer_agent_dropdown() const
+{
+    auto* choice = dynamic_cast<PrinterAgentChoice*>(get_field("printer_agent"));
+    if (!choice || !choice->getWindow())
+        return;
+
+    const auto agents = NetworkAgentFactory::get_registered_printer_agents();
+    if (agents.empty())
+        return;
+
+    // why: rows live on PrinterAgentChoice now; rebuild them from the live registry and re-select the stored id.
+    const std::string selected_agent = wxGetApp().preset_bundle->printers.get_edited_preset()
+                                                 .config.opt_string("printer_agent");
+    choice->reload_rows();
+    choice->set_value(selected_agent, false);
+    this->GetParent()->Layout();
+}
+
 bool Tab::validate_custom_gcodes()
 {
     if (m_type != Preset::TYPE_FILAMENT &&