M8-R18: stabilize Mantine frontend tests

frontend/src/test-utils.test.tsx

@@ -0,0 +1,16 @@
+import { screen } from '@testing-library/react'
+import { useMantineEnv } from '@mantine/core'
+import { describe, expect, it } from 'vitest'
+import { renderWithProviders } from './test-utils'
+
+function MantineEnvironmentProbe() {
+  return <output>{useMantineEnv()}</output>
+}
+
+describe('renderWithProviders', () => {
+  it('uses Mantine test environment', () => {
+    renderWithProviders(<MantineEnvironmentProbe />)
+
+    expect(screen.getByText('test')).toBeInTheDocument()
+  })
+})