feat(manage): add dashboard overview and charts components

- DashboardOverview: stat cards, top contributors, playability distribution,
  and Steam sync health sections
- DashboardCharts: placeholder for future ECharts integration

components/manage/dashboard-charts.tsx

@@ -0,0 +1,7 @@
+"use client";
+
+// Future: ECharts integration for trends over time
+// For now, the overview stats are sufficient
+export function DashboardCharts() {
+  return null;
+}