fix(dashboard): stop negative asset balances inflating net worth (#660)

## Problem

An account of type `checking`/`savings` with a genuinely **negative**
balance was:
- displayed as **positive** on the dashboard account card, and
- **added** to the net worth chart instead of subtracted,

while the **accounts page** rendered the same balance correctly as
negative.

## Root cause

The dashboard used `getAccountSign(type) * Math.abs(value)`. Liabilities
(`credit_card`/`loan`) are stored as positive magnitudes, so `-abs()`
was
correct for them — but for an **asset** holding a negative balance,
`+abs()`
flipped the sign to positive and summed it into net worth. The accounts
page
was correct because it uses the raw signed balance from
`AccountMetricsService`
with no sign transform.

## Fix

- Add `netWorthContribution(type, balance)` in `chart-calculations.ts`:
liabilities subtract their magnitude, assets keep their real sign (so an
  overdrawn asset correctly reduces net worth).
- Dashboard account card (`use-dashboard-data.ts`) now shows the raw
signed
  balance, matching the accounts page.
- Net worth headline total, short/long trend indicators and the
stacked-asset
total (`net-worth-chart.tsx`) use the raw signed value instead of
`Math.abs`.

## Tests

- 3 new unit tests in `chart-calculations.test.ts`, including a
regression case
  for a negative asset balance. Full suite: 37 passing.

resources/js/components/dashboard/net-worth-chart.tsx

@@ -28,8 +28,8 @@ import { useLocale } from '@/hooks/use-locale';
 import { useIsMobile } from '@/hooks/use-mobile';
 import {
     AccountInfo,
-    getAccountSign,
     isLiabilityType,
+    netWorthContribution,
 } from '@/lib/chart-calculations';
 import { SharedData } from '@/types';
 import { formatDayFromDate } from '@/utils/date';
@@ -96,7 +96,7 @@ function calculateTrend(
 
         const account = accounts[id];
 
-        return sum + getAccountSign(account.type) * Math.abs(value);
+        return sum + netWorthContribution(account.type, value);
     }, 0);
 
     const previousTotal = accountIds.reduce((sum, id) => {
@@ -107,7 +107,7 @@ function calculateTrend(
 
         const account = accounts[id];
 
-        return sum + getAccountSign(account.type) * Math.abs(value);
+        return sum + netWorthContribution(account.type, value);
     }, 0);
 
     if (previousTotal === 0) return null;
@@ -316,7 +316,7 @@ export function NetWorthChart({
             chartAccountIds.forEach((id) => {
                 const value = point[id];
                 if (typeof value === 'number') {
-                    totalAssets += Math.abs(value);
+                    totalAssets += value;
                 }
             });
 
@@ -380,7 +380,7 @@ export function NetWorthChart({
                 const value = lastDataPoint[id];
                 if (typeof value === 'number') {
                     const account = includedAccounts[id];
-                    total += getAccountSign(account.type) * Math.abs(value);
+                    total += netWorthContribution(account.type, value);
                 }
             });
         }

resources/js/hooks/use-dashboard-data.ts

@@ -1,5 +1,5 @@
 import { useLocale } from '@/hooks/use-locale';
-import { getAccountSign } from '@/lib/chart-calculations';
+import { netWorthContribution } from '@/lib/chart-calculations';
 import { Account, AccountType, Bank } from '@/types/account';
 import { Category } from '@/types/category';
 import { formatMonthFromYearMonth } from '@/utils/date';
@@ -75,8 +75,10 @@ export function deriveAccountMetrics(
             date: formatMonthFromYearMonth(point.month as string, locale),
             value:
                 typeof point[account.id] === 'number'
-                    ? getAccountSign(account.type) *
-                      Math.abs(point[account.id] as number)
+                    ? netWorthContribution(
+                          account.type,
+                          point[account.id] as number,
+                      )
                     : 0,
             investedAmount:
                 investedKey in point

resources/js/lib/chart-calculations.test.ts

@@ -8,6 +8,7 @@ import {
     getAccountSign,
     isLiabilityType,
     MonthDataPoint,
+    netWorthContribution,
 } from './chart-calculations';
 
 describe('isLiabilityType', () => {
@@ -55,6 +56,23 @@ describe('getAccountSign', () => {
     });
 });
 
+describe('netWorthContribution', () => {
+    it('subtracts the magnitude of liabilities (stored as positive)', () => {
+        expect(netWorthContribution('credit_card', 5000)).toBe(-5000);
+        expect(netWorthContribution('loan', 80000)).toBe(-80000);
+    });
+
+    it('keeps the real sign of assets', () => {
+        expect(netWorthContribution('checking', 288399)).toBe(288399);
+        expect(netWorthContribution('savings', 0)).toBe(0);
+    });
+
+    it('lets a negative asset balance reduce net worth', () => {
+        // Regression: an overdrawn checking account must subtract, not add.
+        expect(netWorthContribution('checking', -23528)).toBe(-23528);
+    });
+});
+
 describe('computeNetWorthSeries', () => {
     const createAccounts = (
         types: Record<string, 'checking' | 'savings' | 'credit_card' | 'loan'>,

resources/js/lib/chart-calculations.ts

@@ -20,6 +20,21 @@ export function getAccountSign(type: AccountType): 1 | -1 {
     return isLiabilityType(type) ? -1 : 1;
 }
 
+/**
+ * Signed contribution of a raw account balance to net worth.
+ *
+ * Liabilities are stored as positive magnitudes, so they always subtract.
+ * Assets keep their real sign, so a genuinely negative asset balance (e.g. an
+ * overdrawn checking account) correctly reduces net worth instead of being
+ * flipped positive and added.
+ */
+export function netWorthContribution(
+    type: AccountType,
+    balance: number,
+): number {
+    return isLiabilityType(type) ? -Math.abs(balance) : balance;
+}
+
 /**
  * Data point representing net worth for a month
  */