Don't crash at exit with a download still in flight

A request started since the last RequestManager::Update (headless never
calls it) sat in newDownloads_, which CancelAll skipped. It was then
destroyed along with the static g_DownloadManager at exit, and its
destructor removed its progress bar from the already destroyed g_OSD:
"mutex lock failed". Seen with a Netconf dialog still downloading the
infra DNS json when a test ended.

CancelAll now takes the new ones too, and runs at shutdown while g_OSD is
still there. The Netconf json request is also let go of when the emulator
shuts down, rather than living on into the next game.

Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>

UI/NativeApp.cpp

@@ -1862,6 +1862,9 @@ void NativeShutdown() {
 
 	__UPnPShutdown();
 
+	// A request finishing while globals are destroyed at exit touches g_OSD, which may be gone by then.
+	g_DownloadManager.CancelAll();
+
 	net::Shutdown();
 
 	g_Discord.Shutdown();