fix: resolve hardcoded light backgrounds breaking dark mode across 5 pages

Replace hardcoded light colors (#e6f9e6, #fde8e8, white, #e9ecef) with
theme-aware alternatives using usePreferencesStore. Affected pages:
- CashFlowForecastPage: forecast row and striped row backgrounds
- MonthlyActualsPage: sticky column backgrounds, borders, section headers
- BudgetsPage: sticky column backgrounds, borders, section headers
- BudgetVsActualPage: income/expense section header backgrounds
- QuarterlyReportPage: income/expense and total row backgrounds

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/pages/reports/BudgetVsActualPage.tsx

@@ -5,6 +5,7 @@ import {
 } from '@mantine/core';
 import { useQuery } from '@tanstack/react-query';
 import api from '../../services/api';
+import { usePreferencesStore } from '../../stores/preferencesStore';
 
 interface BudgetVsActualLine {
   account_id: string;
@@ -46,6 +47,9 @@ const monthFilterOptions = [
 export function BudgetVsActualPage() {
   const [year, setYear] = useState(new Date().getFullYear().toString());
   const [month, setMonth] = useState('');
+  const isDark = usePreferencesStore((s) => s.colorScheme) === 'dark';
+  const incomeBg = isDark ? 'var(--mantine-color-green-9)' : '#e6f9e6';
+  const expenseBg = isDark ? 'var(--mantine-color-red-9)' : '#fde8e8';
 
   const yearOptions = Array.from({ length: 5 }, (_, i) => {
     const y = new Date().getFullYear() - 2 + i;
@@ -92,7 +96,7 @@ export function BudgetVsActualPage() {
 
   const renderSection = (title: string, sectionLines: BudgetVsActualLine[], isExpense: boolean, totalBudget: number, totalActual: number) => (
     <>
-      <Table.Tr style={{ background: isExpense ? '#fde8e8' : '#e6f9e6' }}>
+      <Table.Tr style={{ background: isExpense ? expenseBg : incomeBg }}>
         <Table.Td colSpan={6} fw={700}>{title}</Table.Td>
       </Table.Tr>
       {sectionLines.map((line) => {