CrealityPrint: default Device-tab WebView URL to Mainsail on :4408

K-series printers (K2, K2 Plus, K2 Pro) ship with Mainsail on port
4408. Port 80 hosts only the Creality control/upload API, which
returns 404 for unknown paths and renders as a blank/404 page in
Orca Device tab.

Override CrealityPrint::get_print_host_webui() to default to
http://<host>:4408/ when the user has not explicitly set
print_host_webui, giving K-series owners a complete printer
dashboard in the Device tab out of the box.

src/slic3r/Utils/PrintHost.cpp

@@ -88,6 +88,10 @@ std::string PrintHost::get_print_host_webui(DynamicPrintConfig* config)
         webui_url = ElegooLink::get_print_host_webui(config);
         break;
     }
+    case htCrealityPrint: {
+        webui_url = CrealityPrint::get_print_host_webui(config);
+        break;
+    }
     default: break;
     }