feat: show theoretical balance line in the cash-balance chart

Adds a second, dashed line to the existing balance-history chart that
includes currently open player dues, so managers can see at a glance
how far the actual cash balance lags behind what has been pledged.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

myteamwallet_frontend_modern/src/app/core/team/team-stats-api.spec.ts

@@ -21,7 +21,7 @@ describe('TeamStatsApi', () => {
 
   it('loads the overview stats for a team', () => {
     const stats: TeamOverviewStats = {
-      balanceHistory: [{ month: '2026-07', balance: 125 }],
+      balanceHistory: [{ month: '2026-07', balance: 125, theoreticalBalance: 150 }],
       monthlyFlow: [{ month: '2026-07', income: 50, expense: 12 }],
       topOutstanding: [{ playerId: 3, playerName: 'Alex Muster', balance: 20 }],
     };

myteamwallet_frontend_modern/src/app/features/team/overview/overview.spec.ts

@@ -21,8 +21,8 @@ vi.mock('chart.js', () => ({ Chart: MockChart, registerables: [] }));
 
 const sampleStats: TeamOverviewStats = {
   balanceHistory: [
-    { month: '2026-06', balance: 100 },
+    { month: '2026-06', balance: 100, theoreticalBalance: 100 },
-    { month: '2026-07', balance: 125 },
+    { month: '2026-07', balance: 125, theoreticalBalance: 150 },
   ],
   monthlyFlow: [
     { month: '2026-06', income: 50, expense: 10 },
@@ -193,6 +193,10 @@ describe('Overview', () => {
     );
     expect(balanceChart.type).toBe('line');
     expect(balanceChart.data.labels).toHaveLength(2);
+    expect(balanceChart.data.datasets).toHaveLength(2);
+    expect(balanceChart.data.datasets[0].data).toEqual([100, 125]);
+    expect(balanceChart.data.datasets[1].label).toBe('Theoretisch (inkl. offene Beiträge)');
+    expect(balanceChart.data.datasets[1].data).toEqual([100, 150]);
     expect(flowChart.type).toBe('bar');
     expect(flowChart.data.datasets).toHaveLength(2);
     expect(outstandingChart.type).toBe('bar');

myteamwallet_frontend_modern/src/app/features/team/overview/overview.ts

@@ -21,6 +21,7 @@ import { catchError, distinctUntilChanged, map, switchMap, tap } from 'rxjs/oper
 registerLocaleData(localeDe);
 
 const BALANCE_COLOR = '#4f8f46';
+const THEORETICAL_BALANCE_COLOR = '#1d70b8';
 const INCOME_COLOR = '#4f8f46';
 const EXPENSE_COLOR = '#c1121f';
 
@@ -75,6 +76,15 @@ export class Overview {
           tension: 0.3,
           fill: false,
         },
+        {
+          label: 'Theoretisch (inkl. offene Beiträge)',
+          data: points.map((point) => point.theoreticalBalance),
+          borderColor: THEORETICAL_BALANCE_COLOR,
+          backgroundColor: THEORETICAL_BALANCE_COLOR,
+          borderDash: [6, 4],
+          tension: 0.3,
+          fill: false,
+        },
       ],
     };
   });
@@ -115,7 +125,7 @@ export class Overview {
   protected readonly balanceChartOptions: ChartOptions = {
     responsive: true,
     maintainAspectRatio: false,
-    plugins: { legend: { display: false } },
+    plugins: { legend: { position: 'bottom' } },
   };
 
   protected readonly flowChartOptions: ChartOptions = {

myteamwallet_frontend_modern/src/app/models/team-stats.model.ts

@@ -1,6 +1,7 @@
 export interface BalanceHistoryPoint {
   month: string;
   balance: number;
+  theoreticalBalance: number;
 }
 
 export interface MonthlyFlowPoint {