Move remaining code to app module

There's still some stuff that could potentially go elsewhere but I think it's enough for now.

src/app/proxycontroller.h

@@ -0,0 +1,36 @@
+// SPDX-FileCopyrightText: 2024 Tobias Fella <tobias.fella@kde.org>
+// SPDX-License-Identifier: LGPL-2.0-or-later
+
+#pragma once
+
+#include <QObject>
+#include <QQmlEngine>
+
+class ProxyController : public QObject
+{
+    Q_OBJECT
+    QML_ELEMENT
+    QML_SINGLETON
+
+public:
+    static ProxyController &instance()
+    {
+        static ProxyController _instance;
+        return _instance;
+    }
+    static ProxyController *create(QQmlEngine *engine, QJSEngine *)
+    {
+        engine->setObjectOwnership(&instance(), QQmlEngine::CppOwnership);
+        return &instance();
+    }
+
+    /**
+     * @brief Sets the QNetworkProxy for the application.
+     *
+     * @sa QNetworkProxy::setApplicationProxy
+     */
+    Q_INVOKABLE void setApplicationProxy();
+
+private:
+    explicit ProxyController(QObject *parent = nullptr);
+};