fix: move new Set<string>() default prop to module-level constant in CandleChart

Move the inline 'new Set<string>()' default prop to a module-level constant
(EMPTY_STRING_SET) to prevent unnecessary Set instance creation on every render,
which could cause unnecessary re-renders in children or effects that depend on
reference equality.

src/components/CandleChart.tsx

@@ -59,6 +59,9 @@ const PREDEFINED_LABEL_COLORS = [
   '#f97316', // orange
 ];
 
+// === Default Props ===
+const EMPTY_STRING_SET = new Set<string>();
+
 interface DrawingState {
   tool: 'line' | 'rectangle';
   firstPoint: { time: Time; price: number };
@@ -173,7 +176,7 @@ const CandleChart = forwardRef<CandleChartHandle, CandleChartProps>(
     perCandlePredictions = [],
     predictionSpans = [],
     confidenceThreshold = 0.5,
-    selectedLabels = new Set<string>(),
+    selectedLabels = EMPTY_STRING_SET,
     modelInfo = null,
     predictionSummary = null,
     showOnlyDisagreements = false,