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/App.tsx

@@ -25,6 +25,7 @@ import { YearEndPage } from './pages/reports/YearEndPage';
 import { SettingsPage } from './pages/settings/SettingsPage';
 import { AdminPage } from './pages/admin/AdminPage';
 import { AssessmentGroupsPage } from './pages/assessment-groups/AssessmentGroupsPage';
+import { CashFlowForecastPage } from './pages/cash-flow/CashFlowForecastPage';
 
 function ProtectedRoute({ children }: { children: React.ReactNode }) {
   const token = useAuthStore((s) => s.token);
@@ -114,6 +115,7 @@ export function App() {
         <Route path="investments" element={<InvestmentsPage />} />
         <Route path="capital-projects" element={<CapitalProjectsPage />} />
         <Route path="assessment-groups" element={<AssessmentGroupsPage />} />
+        <Route path="cash-flow" element={<CashFlowForecastPage />} />
         <Route path="reports/balance-sheet" element={<BalanceSheetPage />} />
         <Route path="reports/income-statement" element={<IncomeStatementPage />} />
         <Route path="reports/budget-vs-actual" element={<BudgetVsActualPage />} />