Highlight table row when hovering spending chart legend items

Add legendMouseOver/legendMouseOut events to sync legend hover
with the category table highlighting.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/views/reports/ReportsView.vue

@@ -347,6 +347,13 @@ const spendingChartOptions = computed(() => ({
       dataPointMouseLeave: () => {
         highlightedCategory.value = null
       },
+      legendMouseOver: (_ctx: any, _opts: any, config: any) => {
+        const labels = spending.value?.categories.map(c => c.categoryName) ?? []
+        highlightedCategory.value = labels[config.seriesIndex] ?? null
+      },
+      legendMouseOut: () => {
+        highlightedCategory.value = null
+      },
     },
   },
   labels: spending.value?.categories.map(c => c.categoryName) ?? [],