[Settings] Delete update channels at shutdown

SettingsCache owns the stable and beta update channels as raw pointers, and
each owns a QNetworkAccessManager, so the process leaves QtNetwork objects
alive past QApplication teardown.

Delete them from a QCoreApplication::aboutToQuit handler instead of the
destructor: SettingsCache is a Q_GLOBAL_STATIC, so ~SettingsCache() runs
during static teardown, after QApplication is gone, and destroying the
network managers at that point is the post-application window that ASan
reports. The hook is installed on the first instance() call that has an
application object, since the first access may predate it.

cockatrice/src/client/settings/cache_settings.cpp

@@ -6,6 +6,7 @@
 #include "shortcuts_settings.h"
 
 #include <QApplication>
+#include <QCoreApplication>
 #include <QDebug>
 #include <QDir>
 #include <QFile>
@@ -47,7 +48,32 @@ Q_GLOBAL_STATIC(SettingsCache, settingsCache)
 
 SettingsCache &SettingsCache::instance()
 {
-    return *settingsCache; // returns a QT managed singleton reference
+    SettingsCache &cache = *settingsCache; // returns a QT managed singleton reference
+    // The release channels own QNetworkAccessManagers, which must be destroyed
+    // while the application object is alive. ~SettingsCache() runs during
+    // static teardown, after QApplication is gone, so hook the application's
+    // shutdown instead. Retried here because the first access may predate the
+    // application object.
+    cache.installShutdownHook();
+    return cache;
+}
+
+void SettingsCache::installShutdownHook()
+{
+    if (shutdownHookInstalled) {
+        return;
+    }
+    QCoreApplication *application = QCoreApplication::instance();
+    if (application == nullptr) {
+        return;
+    }
+    connect(application, &QCoreApplication::aboutToQuit, this, [this] {
+        for (ReleaseChannel *releaseChannel : std::as_const(releaseChannels)) {
+            delete releaseChannel;
+        }
+        releaseChannels.clear();
+    });
+    shutdownHookInstalled = true;
 }
 
 QString SettingsCache::getDataPath()

cockatrice/src/client/settings/cache_settings.h

@@ -91,8 +91,13 @@ private:
     QString themeName;
 
     QList<ReleaseChannel *> releaseChannels;
+    bool shutdownHookInstalled = false;
     bool isPortableBuild;
 
+    // Connects the aboutToQuit handler that deletes the release channels while
+    // the application object is still alive; a no-op before it exists.
+    void installShutdownHook();
+
     [[nodiscard]] QString getSafeConfigPath(QString configEntry, QString defaultPath) const;
     [[nodiscard]] QString getSafeConfigFilePath(QString configEntry, QString defaultPath) const;
     void loadPaths();