fix: optimization: PVS-Studio warning V831

Replaced 'at()' method with 'operator[]' to improve performance.

The 'at()' method performs bounds checking, which can introduce overhead. Using 'operator[]' bypasses this check and can improve performance when you are certain that the index is within bounds.

Reported by: PVS-Studio
Signed-off-by: Vyacheslav Ivanov <islavaivanov76@gmail.com>

src/graphics/ui/gui_xml.cpp

@@ -286,7 +286,7 @@ static std::shared_ptr<UINode> readButton(UiXmlReader& reader, const xml::xmlele
 
     std::shared_ptr<Button> button;
     auto& elements = element->getElements();
-    if (!elements.empty() && elements.at(0)->getTag() != "#") {
+    if (!elements.empty() && elements[0]->getTag() != "#") {
         auto inner = reader.readUINode(element->getElements().at(0));
         if (inner != nullptr) {
             button = std::make_shared<Button>(inner, padding);