Add Cashflow feature to frontend-web

- Introduced CashflowPage component for managing income and expenses, including detailed summaries and transaction tracking.
- Updated App component to include routing for the new CashflowPage.
- Enhanced Layout component with navigation for Cashflow.
- Created Redux slice for cashflow management, including actions for income sources, expenses, and transactions.
- Integrated mock data for initial cashflow setup to facilitate development and testing.

frontend-web/src/components/Layout.tsx

@@ -1,8 +1,9 @@
 import {NavLink, Outlet} from 'react-router-dom';
-import {TrendingUp, CreditCard, FileText, Users} from 'lucide-react';
+import {TrendingUp, CreditCard, FileText, Users, ArrowLeftRight} from 'lucide-react';
 
 const navItems = [
   {to: '/', label: 'Net Worth', icon: TrendingUp},
+  {to: '/cashflow', label: 'Cashflow', icon: ArrowLeftRight},
   {to: '/debts', label: 'Debts', icon: CreditCard},
   {to: '/invoices', label: 'Invoices', icon: FileText},
   {to: '/clients', label: 'Clients', icon: Users},