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/elements/Label.cpp

@@ -145,7 +145,7 @@ uint Label::getLineByYOffset(int offset) const {
 
 uint Label::getLineByTextIndex(size_t index) const {
     for (size_t i = 0; i < cache.lines.size(); i++) {
-        if (cache.lines.at(i).offset > index) {
+        if (cache.lines[i].offset > index) {
             return i-1;
         }
     }
@@ -195,7 +195,7 @@ void Label::draw(const DrawContext* pctx, Assets* assets) {
 
     if (multiline) {
         for (size_t i = 0; i < cache.lines.size(); i++) {
-            auto& line = cache.lines.at(i);
+            auto& line = cache.lines[i];
             size_t offset = line.offset;
             std::wstring_view view(text.c_str()+offset, text.length()-offset);
             if (i < cache.lines.size()-1) {