feat: add Buchhaltung module with fiscal years, budget accounts, transactions, and approval workflow

frontend/src/components/admin/PermissionMatrixTab.tsx

@@ -118,6 +118,11 @@ const PERMISSION_SUB_GROUPS: Record<string, Record<string, string[]>> = {
     'Verwaltung': ['manage_templates'],
     'Widget': ['widget'],
   },
+  buchhaltung: {
+    'Ansicht':        ['view', 'widget'],
+    'Transaktionen':  ['create', 'edit', 'delete', 'export'],
+    'Verwaltung':     ['manage_accounts', 'manage_settings'],
+  },
   admin: {
     'Allgemein': ['view', 'write'],
     'Services': ['view_services', 'edit_services'],

frontend/src/components/shared/Sidebar.tsx

@@ -30,6 +30,7 @@ import {
   BookOnline,
   Forum,
   AssignmentTurnedIn,
+  AccountBalance as AccountBalanceIcon,
 } from '@mui/icons-material';
 import { useNavigate, useLocation } from 'react-router-dom';
 import { useQuery } from '@tanstack/react-query';
@@ -143,6 +144,17 @@ const baseNavigationItems: NavigationItem[] = [
     path: '/checklisten',
     permission: 'checklisten:view',
   },
+  {
+    text: 'Buchhaltung',
+    icon: <AccountBalanceIcon />,
+    path: '/buchhaltung',
+    subItems: [
+      { text: 'Übersicht',       path: '/buchhaltung?tab=0' },
+      { text: 'Transaktionen',   path: '/buchhaltung?tab=1' },
+      { text: 'Konten',          path: '/buchhaltung?tab=2' },
+    ],
+    permission: 'buchhaltung:view',
+  },
   {
     text: 'Issues',
     icon: <BugReport />,