Work around startup UI freeze caused by QtTextToSpeech

This merge request works around an annoying startup hang that I introduced when adding text-to-speech to NeoChat. The previous implementation was a QML singleton that used the `TextToSpeech` QML component. Unfortunately that component blocks the UI thread when first loading it, while it connects to speech-dispatcher.

This MR just rewrites that singleton in C++, and moves initialization of QtTextToSpeech to the first time you read a message aloud. It doesn't fix the performance problem, but it at least stops it from affecting startup.

In the future, I'd like to move speech operations to a background thread to completely mitigate the initialization freeze.

src/app/texttospeechhelper.h

@@ -0,0 +1,21 @@
+// SPDX-FileCopyrightText: 2025 Ritchie Frodomar <ritchie@kde.org>
+// SPDX-License-Identifier: GPL-2.0-only OR GPL-3.0-only OR LicenseRef-KDE-Accepted-GPL
+
+#pragma once
+
+#include <QObject>
+#include <QQmlEngine>
+#include <QtTextToSpeech>
+
+class TextToSpeechHelper : public QObject
+{
+    Q_OBJECT
+    QML_ELEMENT
+    QML_SINGLETON
+
+public:
+    Q_INVOKABLE void speak(const QString &textToSpeak);
+
+private:
+    QTextToSpeech *m_speech = nullptr;
+};