fix: add React Router v7 future flags and defensive null guard for wartungslog

frontend/src/main.tsx

@@ -17,7 +17,7 @@ const queryClient = new QueryClient({
 
 ReactDOM.createRoot(document.getElementById('root')!).render(
   <React.StrictMode>
-    <BrowserRouter>
+    <BrowserRouter future={{ v7_startTransition: true, v7_relativeSplatPath: true }}>
       <ThemeProvider theme={lightTheme}>
         <CssBaseline />
         <QueryClientProvider client={queryClient}>

frontend/src/pages/FahrzeugDetail.tsx

@@ -628,7 +628,7 @@ function FahrzeugDetail() {
         <TabPanel value={activeTab} index={1}>
           <WartungTab
             fahrzeugId={vehicle.id}
-            wartungslog={vehicle.wartungslog}
+            wartungslog={vehicle.wartungslog ?? []}
             onAdded={fetchVehicle}
             canWrite={canChangeStatus}
           />