fix(MouseHighlighter): clean up old shapes to prevent memory leak in long sessions

After appending a new shape to the composition, check if the shapes
collection exceeds 500 entries. If so, remove the oldest 100 shapes
which are guaranteed to be fully faded out by that point.

This prevents unbounded memory growth during long drawing sessions.

Fixes #46964

src/modules/MouseUtils/MouseHighlighter/MouseHighlighter.cpp

@@ -217,8 +217,18 @@ void Highlighter::AddDrawingPoint(MouseButton button)
 
     m_shape.Shapes().Append(circleShape);
 
-    // TODO: We're leaking shapes for long drawing sessions.
-    // Perhaps add a task to the Dispatcher every X circles to clean up.
+    // Remove old shapes to prevent unbounded memory growth in long sessions.
+    // Shapes older than the fade duration are fully transparent and safe to remove.
+    constexpr uint32_t maxShapes = 500;
+    constexpr uint32_t removeCount = 100;
+    auto shapes = m_shape.Shapes();
+    if (shapes.Size() > maxShapes)
+    {
+        for (uint32_t i = 0; i < removeCount && shapes.Size() > 0; ++i)
+        {
+            shapes.RemoveAt(0);
+        }
+    }
 
     // Get back on top in case other Window is now the topmost.
     // HACK: Draw with 1 pixel off. Otherwise, Windows glitches the task bar transparency when a transparent window fill the whole screen.