Add Phase 4 Cash Flow Visualization with forecast endpoint and Recharts chart

New feature: Cash Flow page under Financials showing stacked area chart of
operating/reserve cash and investment balances over time. Backend forecast
endpoint integrates assessment income schedules, budget expenses, capital
project costs, and investment maturities to project 24+ months forward.
Historical months show actual journal entry balances; future months are
projected. Includes Operating/Reserve/All fund filter, 12-month sliding
window navigation, forecast reference line, and monthly detail table.

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

frontend/src/components/layout/Sidebar.tsx

@@ -15,6 +15,7 @@ import {
   IconSettings,
   IconCrown,
   IconCategory,
+  IconChartAreaLine,
 } from '@tabler/icons-react';
 import { useAuthStore } from '../../stores/authStore';
 
@@ -28,6 +29,7 @@ const navSections = [
     label: 'Financials',
     items: [
       { label: 'Accounts', icon: IconListDetails, path: '/accounts' },
+      { label: 'Cash Flow', icon: IconChartAreaLine, path: '/cash-flow' },
       { label: 'Budgets', icon: IconReportAnalytics, path: '/budgets/2026' },
     ],
   },