M8-R18: stabilize Mantine frontend tests

frontend/src/test-utils.tsx

@@ -51,7 +51,7 @@ export function renderWithProviders(ui: ReactNode, options: RenderOptions = {})
 
   function Wrapper() {
     return (
-      <MantineProvider>
+      <MantineProvider env="test">
         <QueryClientProvider client={queryClient}>
           <MemoryRouter initialEntries={entries}>
             <Routes>