feat(overview): add KPI charts to team overview page

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>

myteamwallet_frontend_modern/src/app/shared/chart-canvas/chart-canvas.scss

@@ -0,0 +1,10 @@
+:host {
+  display: block;
+  position: relative;
+  width: 100%;
+  height: 100%;
+}
+canvas {
+  width: 100% !important;
+  height: 100% !important;
+}