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/cash-flow/CashFlowForecastPage.tsx

@@ -8,6 +8,7 @@ import {
   IconArrowLeft, IconArrowRight, IconCalendar,
 } from '@tabler/icons-react';
 import { useQuery } from '@tanstack/react-query';
+import { usePreferencesStore } from '../../stores/preferencesStore';
 import {
   AreaChart, Area, XAxis, YAxis, CartesianGrid,
   Tooltip as RechartsTooltip, ResponsiveContainer, Legend,
@@ -79,6 +80,7 @@ export function CashFlowForecastPage() {
   const now = new Date();
   const currentYear = now.getFullYear();
   const currentMonth = now.getMonth() + 1;
+  const isDark = usePreferencesStore((s) => s.colorScheme) === 'dark';
 
   // Filter: All, Operating, Reserve
   const [fundFilter, setFundFilter] = useState<string>('all');
@@ -418,10 +420,10 @@ export function CashFlowForecastPage() {
                   <tr
                     key={d.month}
                     style={{
-                      borderBottom: '1px solid var(--mantine-color-gray-2)',
+                      borderBottom: `1px solid ${isDark ? 'var(--mantine-color-dark-4)' : 'var(--mantine-color-gray-2)'}`,
                       backgroundColor: d.is_forecast
-                        ? 'var(--mantine-color-orange-0)'
-                        : i % 2 === 0 ? 'transparent' : 'var(--mantine-color-gray-0)',
+                        ? (isDark ? 'var(--mantine-color-orange-9)' : 'var(--mantine-color-orange-0)')
+                        : i % 2 === 0 ? 'transparent' : (isDark ? 'var(--mantine-color-dark-5)' : 'var(--mantine-color-gray-0)'),
                     }}
                   >
                     <td style={{ padding: '6px 12px', fontWeight: 500 }}>{d.month}</td>