feat(insights): add VelocityChart grouped BarChart component

Renders DONE-task counts per sprint per product as grouped bars with
SERIES_COLORS, a dotted average ReferenceLine, and an empty-state for
<2 completed sprints.

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

app/(app)/insights/components/velocity-chart.tsx

@@ -0,0 +1,54 @@
+'use client'
+
+import {
+  BarChart,
+  Bar,
+  XAxis,
+  YAxis,
+  Tooltip,
+  Legend,
+  ReferenceLine,
+  ResponsiveContainer,
+} from 'recharts'
+import type { VelocityData } from '@/lib/insights/velocity'
+import { SERIES_COLORS } from '@/lib/chart-colors'
+
+interface Props {
+  data: VelocityData
+}
+
+export function VelocityChart({ data }: Props) {
+  const { sprints, productNames } = data
+
+  if (sprints.length < 2) {
+    return (
+      <p className="text-muted-foreground text-sm">
+        Velocity wordt zichtbaar na 2+ voltooide sprints
+      </p>
+    )
+  }
+
+  const chartData = sprints.map((s, i) => {
+    const row: Record<string, string | number> = { sprintLabel: `S${i + 1}` }
+    row[s.productName] = s.doneCount
+    return row
+  })
+
+  const totalDone = sprints.reduce((sum, s) => sum + s.doneCount, 0)
+  const avg = Math.round((totalDone / sprints.length) * 10) / 10
+
+  return (
+    <ResponsiveContainer width="100%" height={260}>
+      <BarChart data={chartData}>
+        <XAxis dataKey="sprintLabel" tick={{ fontSize: 11 }} />
+        <YAxis allowDecimals={false} tick={{ fontSize: 11 }} />
+        <Tooltip />
+        <Legend />
+        <ReferenceLine y={avg} stroke="var(--muted-foreground)" strokeDasharray="3 3" />
+        {productNames.map((p, i) => (
+          <Bar key={p.id} dataKey={p.name} fill={SERIES_COLORS[i % SERIES_COLORS.length]} />
+        ))}
+      </BarChart>
+    </ResponsiveContainer>
+  )
+}