money: day-by-day forecast with real payment dates

The wealth forecast now runs per day instead of per month, so it shows the
same sawtooth as the actual history:

- recurring items hit on the day the bank history shows (most common day of
  the last four months, clamped to month end), else their start date
- "every N weeks" items (Basic-Fit, haircut) run on an exact N-day cadence
  from the last charge; yearly items on their renewal date
- salary and its extras (holiday pay in May, 13th month in November, bonus,
  settlement) land on the payday from history (contract 25th as fallback),
  moved to the Friday before a weekend; toeslagen move to the Monday after
- variable budgets are spread per day, the vacation budget over Jul-Aug
- excess above the buffer is invested the day after payday; returns
  compound daily
- long horizons stay fast (~35 ms for 100 years): the chart thins each
  pixel bucket to its low and high

src/lib/money-tracker/charts.test.ts

@@ -3,6 +3,7 @@ import {
 	accountBalances,
 	accountColor,
 	balanceSeries,
+	decimateSeries,
 	forecastSeries,
 	parseHistoryRange,
 	gridFor,
@@ -119,7 +120,18 @@ describe('money tracker calculations', () => {
 			settings,
 			'2026-09-23'
 		);
-		expect(series[0].points.map((point) => point.value)).toEqual([100000, 100000, 160000, 210244]);
+		const on = (index: number, date: string) =>
+			series[index].points.find((point) => point.date === date)?.value;
+		expect(series[0].points).toHaveLength(69);
+		expect(
+			['2026-09-23', '2026-09-30', '2026-10-31', '2026-11-30'].map((date) => on(0, date))
+		).toEqual([100000, 100000, 160000, 210244]);
+		// Day by day: the monthly item lands on the 1st and the estimate on the 15th.
+		expect(on(1, '2026-10-01')).toBe(140000);
+		expect(on(1, '2026-10-15')).toBe(160000);
+		// Excess above the reserve moves into investments at the end of the month without a payday.
+		expect(on(1, '2026-10-31')).toBe(100000);
+		expect(on(2, '2026-10-31')).toBe(60000);
 		expect(series[1].points.at(-1)?.value).toBe(100000);
 		expect(series[2].points.at(-1)?.value).toBe(110244);
 	});
@@ -151,9 +163,10 @@ describe('money tracker calculations', () => {
 			},
 			'2026-09-23'
 		);
-		expect(projection[0].points.map((point) => point.value)).toEqual([
-			40054, 40054, 167191, 294602
-		]);
+		const total = new Map(projection[0].points.map((point) => [point.date, point.value]));
+		expect(
+			['2026-09-23', '2026-09-30', '2026-10-31', '2026-11-30'].map((date) => total.get(date))
+		).toEqual([40054, 40054, 167191, 294602]);
 		expect(projection[1].points.at(-1)?.value).toBe(100000);
 		expect(projection[2].points.at(-1)?.value).toBe(194602);
 	});
@@ -373,4 +386,62 @@ describe('history range', () => {
 			'2120'
 		]);
 	});
+
+	it('draws a day-by-day sawtooth: salary on payday, costs on their days, invest the day after', () => {
+		const projection = wealthProjection(
+			[
+				{ id: 1, kind: 'spendable', openingBalanceCents: 0 },
+				{ id: 2, kind: 'investment', openingBalanceCents: 0 }
+			],
+			new Map([
+				[1, 150000],
+				[2, 0]
+			]),
+			[],
+			[],
+			[
+				{ name: 'Cargill salary', amountCents: 300000, cadence: 'monthly', accountId: 1 },
+				{ name: 'Spotify', amountCents: -1199, cadence: 'monthly', accountId: 1 }
+			],
+			{
+				investmentAccountId: 2,
+				reserveCents: 100000,
+				annualReturnPercent: '0.00',
+				contributionMode: 'excess',
+				fixedMonthlyCents: 0,
+				includeRecurring: true,
+				endDate: '2026-12-31'
+			},
+			'2026-10-26',
+			null,
+			[
+				{ date: '2026-10-21', amountCents: 298653, description: 'Salaris Cargill' },
+				{ date: '2026-10-24', amountCents: -1199, description: 'Spotify AB' }
+			]
+		);
+		const cash = new Map(projection[1].points.map((point) => [point.date, point.value]));
+		const invested = new Map(projection[2].points.map((point) => [point.date, point.value]));
+		expect(cash.get('2026-11-19')).toBe(150000);
+		// 21 Nov is a Saturday: paid on Friday the 20th, invested above the reserve on the 21st.
+		expect(cash.get('2026-11-20')).toBe(450000);
+		expect(cash.get('2026-11-21')).toBe(100000);
+		expect(invested.get('2026-11-21')).toBe(350000);
+		expect(cash.get('2026-11-24')).toBe(100000 - 1199);
+		expect(cash.get('2026-12-21')).toBe(100000 - 1199 + 300000);
+	});
+
+	it('thins long daily series to each bucket’s low and high, keeping both ends', () => {
+		const points = Array.from({ length: 3650 }, (_, index) => ({
+			date: new Date(Date.UTC(2026, 0, 1 + index)).toISOString().slice(0, 10),
+			value: index % 30 === 0 ? 1000 : index % 30 === 15 ? -1000 : 0
+		}));
+		const series = [{ label: 'Total', color: '#000', points }];
+		const [thin] = decimateSeries(series, 100);
+		expect(thin.points.length).toBeLessThanOrEqual(202);
+		expect(thin.points[0]).toEqual(points[0]);
+		expect(thin.points.at(-1)).toEqual(points.at(-1));
+		expect(Math.max(...thin.points.map((point) => point.value))).toBe(1000);
+		expect(Math.min(...thin.points.map((point) => point.value))).toBe(-1000);
+		expect(decimateSeries(series, 5000)).toBe(series);
+	});
 });