Prepend system message from settings to API payload

Add the system message (if configured and non‑empty) to the beginning of
the JSON array that is sent to the LlamaCPP server.

The new entry uses the role "system" and contains the configured text.

This ensures the server receives the system context before any user
messages, improving generation behaviour without changing the existing
message flow.

Author: cristianadam

llamachatmanager.cpp

@@ -543,6 +543,15 @@ LlamaCppServerProps ChatManager::serverProps() const
 QJsonArray ChatManager::normalizeMsgsForAPI(const QVector<Message> &msgs)
 {
     QJsonArray res;
+
+    const QString sysMsgText = LlamaCpp::settings().systemMessage.value();
+    if (!sysMsgText.trimmed().isEmpty()) {
+        QJsonObject sys;
+        sys["role"] = "system";
+        sys["content"] = sysMsgText;
+        res.append(sys);
+    }
+
     for (const Message &msg : msgs) {
         if (msg.role != "user" || msg.extra.isEmpty()) {
             QJsonObject out;