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/package-lock.json

@@ -17,6 +17,7 @@
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@@ -2115,6 +2116,12 @@
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@@ -4639,6 +4646,18 @@
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