Adds three chart.js-backed KPI cards (Kassenstand-Verlauf, Einnahmen &
Ausgaben, Top-10 offene Beitraege) to the existing Uebersicht page,
consuming the new GET teams/:id/overview/stats endpoint via a new
TeamStatsApi service. Introduces a small reusable ChartCanvas shared
component that wraps the Chart.js instance lifecycle via @Input()/
ngOnChanges, following this codebase's existing input-decorator
convention rather than effect().
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
