refactor(js): use shared formatMonthFromYearMonth in dashboard (#482)

## What

`use-dashboard-data.ts` had a private `formatMonth()` duplicating
`utils/date.ts` `formatMonthFromYearMonth()`.

Removed the local copy; dashboard now uses the shared util.

**Tiny visual change:** non-current-year labels on dashboard net-worth
chart render `Jan '26` instead of `Jan 26` — now consistent with
cashflow-trend and account-balance charts which already use the shared
util.

## Stats

- **-14 / +2 lines**

## Checks

- `bun run test` — 154 passed
- `bun run lint` — clean (1 pre-existing warning)

Part of duplication-removal series (#475–#481).

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

@@ -2,6 +2,7 @@ import { useLocale } from '@/hooks/use-locale';
 import { getAccountSign } from '@/lib/chart-calculations';
 import { Account, AccountType, Bank } from '@/types/account';
 import { Category } from '@/types/category';
+import { formatMonthFromYearMonth } from '@/utils/date';
 import { format, subDays, subMonths } from 'date-fns';
 import { useCallback, useEffect, useState } from 'react';
 
@@ -66,7 +67,7 @@ export function deriveAccountMetrics(
     return Object.values(accounts).map((account) => {
         const investedKey = account.id + '_invested';
         const history = data.map((point) => ({
-            date: formatMonth(point.month as string, locale),
+            date: formatMonthFromYearMonth(point.month as string, locale),
             value:
                 typeof point[account.id] === 'number'
                     ? getAccountSign(account.type) *
@@ -100,20 +101,6 @@ export function deriveAccountMetrics(
     });
 }
 
-function formatMonth(yearMonth: string, locale = 'en-US'): string {
-    const [year, month] = yearMonth.split('-');
-    const date = new Date(parseInt(year), parseInt(month) - 1);
-
-    const isCurrentYear = date.getFullYear() === new Date().getFullYear();
-
-    return date.toLocaleDateString(
-        locale,
-        isCurrentYear
-            ? { month: 'short' }
-            : { year: '2-digit', month: 'short' },
-    );
-}
-
 export function useDashboardData(): DashboardData & { refetch: () => void } {
     const locale = useLocale();
     const [data, setData] = useState<Omit<DashboardData, 'isLoading'>>({