feat: Add invoice management menu and template editor for admin

- Add 'Rechnung' menu with sub-items 'Verwalten' and 'Template' in admin sidebar
- Create InvoiceTemplatePage with drag-and-drop editor for invoice templates
  - Includes invoice-specific elements (header, customer data, issuer info,
    invoice items, totals, payment terms, bank details)
  - Supports PDF preview and download
  - API integration for saving/loading templates (/admin/invoice-template)
- Create InvoiceManagementPage as placeholder for invoice overview
- Add routes for /admin/rechnung/verwalten and /admin/rechnung/template
- Update page titles in AppShell for new routes

frontend/src/App.tsx

@@ -15,6 +15,8 @@ import SearchFarmerPage from "./pages/SearchFarmerPage";
 import SearchCalendarPage from "./pages/SearchCalendarPage";
 import UserManagementPage from "./pages/UserManagementPage";
 import ReportTemplatePage from "./pages/ReportTemplatePage";
+import InvoiceTemplatePage from "./pages/InvoiceTemplatePage";
+import InvoiceManagementPage from "./pages/InvoiceManagementPage";
 
 function ProtectedRoutes() {
   const { user, ready } = useSession();
@@ -45,6 +47,8 @@ function ProtectedRoutes() {
         <Route path="/admin/medikamente" element={<AdministrationPage />} />
         <Route path="/admin/erreger" element={<AdministrationPage />} />
         <Route path="/admin/antibiogramm" element={<AdministrationPage />} />
+        <Route path="/admin/rechnung/verwalten" element={<InvoiceManagementPage />} />
+        <Route path="/admin/rechnung/template" element={<InvoiceTemplatePage />} />
         <Route path="/search" element={<Navigate to="/search/probe" replace />} />
         <Route path="/search/probe" element={<SearchPage />} />
         <Route path="/search/landwirt" element={<SearchFarmerPage />} />