feat: single-series bar chart uses different color per category

Each bar gets a different color from the palette, making color
palette changes immediately visible for all chart configurations.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/adapters/gateways/EChartsOptionBuilder/barOptionBuilder.ts

@@ -211,10 +211,10 @@ export function buildBarOption(
     option.series = [
       {
         type: 'bar',
-        data: seriesData,
-        itemStyle: {
-          color: style.colors[0],
-        },
+        data: seriesData.map((val, idx) => ({
+          value: val,
+          itemStyle: { color: style.colors[idx % style.colors.length] },
+        })),
         label,
       },
     ];