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.json

@@ -21,6 +21,7 @@
     "@angular/platform-browser": "^21.2.0",
     "@angular/router": "^21.2.0",
     "@angular/service-worker": "^21.2.0",
+    "chart.js": "^4.5.1",
     "qrcode": "^1.5.4",
     "rxjs": "~7.8.0",
     "tslib": "^2.3.0"