fix: use Highcharts default values for legend symbol reset

undefined doesn't properly reset merged options. Use the actual
Highcharts defaults: symbolWidth:16, symbolHeight:12, symbolPadding:5.

Author: austenstone

src/components/charts/CostBreakdownChart.tsx

@@ -104,7 +104,7 @@ export function CostBreakdownChart({ stackField = 'model', metricOptions }: Cost
       plotOptions: { column: { stacking: 'normal' } },
       legend: stackField === 'sku'
         ? { symbolWidth: 0, symbolHeight: 0, symbolPadding: 0 }
-        : { symbolWidth: undefined, symbolHeight: undefined, symbolPadding: undefined },
+        : { symbolWidth: 16, symbolHeight: 12, symbolPadding: 5 },
       series,
     };
   }, [activeReport, timeBucket, visibleRows, stackField, activeMetric, dataField]);

src/components/charts/ModelBreakdownChart.tsx

@@ -176,7 +176,7 @@ export function GroupBreakdownChart({ stackField = 'model', metricOptions }: Gro
       plotOptions: { bar: { stacking: 'normal' } },
       legend: stackField === 'sku'
         ? { symbolWidth: 0, symbolHeight: 0, symbolPadding: 0 }
-        : { symbolWidth: undefined, symbolHeight: undefined, symbolPadding: undefined },
+        : { symbolWidth: 16, symbolHeight: 12, symbolPadding: 5 },
       series,
     };
   }, [activeReport, groupByColumn, stackField, visibleRows, hiddenGroups, toggleGroup, activeMetric, dataField]);

src/components/charts/TimeSeriesChart.tsx

@@ -182,7 +182,7 @@ export function TimeSeriesChart({ metricOptions }: { metricOptions?: MetricOptio
       series,
       legend: groupByColumn === 'sku'
         ? { symbolWidth: 0, symbolHeight: 0, symbolPadding: 0 }
-        : { symbolWidth: undefined, symbolHeight: undefined, symbolPadding: undefined },
+        : { symbolWidth: 16, symbolHeight: 12, symbolPadding: 5 },
       chart: { height: 400 },
     };
   }, [activeReport, groupByColumn, timeBucket, visibleRows, lineMode, rollingWindow, dataField, activeMetric]);