[Network] Add a configured QNetworkAccessManager factory (#7628)

* [Network] Add a configured QNetworkAccessManager factory

Every HTTP caller built its own QNetworkAccessManager, so the system proxy
and a transfer timeout were easy to omit. Add NetworkManagerFactory::create()
as the single place that applies the process-wide proxy configuration and a
transfer timeout, and use it at all 14 construction sites.

Per-site settings such as disk caches and manual redirect policies stay with
their callers.

* [Tests] Link libcockatrice_network into the tests that compile its headers

release_channel.cpp and card_picture_loader_worker.cpp now include
libcockatrice/network/network_manager.h, but the test targets that compile
them only linked libcockatrice_settings, so the include directory never
reached them and every platform failed with "file not found":
loader_local_matching_test and picture_loader_benchmark_test.

libcockatrice_network is an INTERFACE library, so linking it costs these
targets nothing but hands over the include path.

---------

Co-authored-by: Lukas Brübach <Bruebach.Lukas@bdosecurity.de>

libcockatrice_network/libcockatrice/network/network_manager.h

@@ -0,0 +1,38 @@
+#ifndef COCKATRICE_NETWORK_NETWORK_MANAGER_H
+#define COCKATRICE_NETWORK_NETWORK_MANAGER_H
+
+#include <QNetworkAccessManager>
+#include <QNetworkProxyFactory>
+#include <mutex>
+
+/**
+ * @file network_manager.h
+ * @ingroup Network
+ * @brief Creates QNetworkAccessManagers with the application-wide baseline.
+ *
+ * Every manager in the client should start from NetworkManagerFactory::create()
+ * so features that apply to all HTTP traffic (the system proxy, a transfer
+ * timeout) cannot be forgotten in one of the many call sites. Feature-specific
+ * setup such as a disk cache or a manual redirect policy stays with the caller.
+ */
+namespace NetworkManagerFactory
+{
+/**
+ * @brief Returns a QNetworkAccessManager configured for Cockatrice.
+ *
+ * The manager is parented to @p parent. The system proxy is enabled once,
+ * process-wide, the first time this function is called. Transfers use Qt's
+ * default timeout instead of waiting forever.
+ */
+inline QNetworkAccessManager *create(QObject *parent = nullptr)
+{
+    static std::once_flag systemProxyFlag;
+    std::call_once(systemProxyFlag, [] { QNetworkProxyFactory::setUseSystemConfiguration(true); });
+
+    auto *manager = new QNetworkAccessManager(parent);
+    manager->setTransferTimeout();
+    return manager;
+}
+} // namespace NetworkManagerFactory
+
+#endif // COCKATRICE_NETWORK_NETWORK_MANAGER_H