Add storage balloon pre-check with actionable error messages because browsers lie about storage space availability.

src/app/App.test.jsx

@@ -1,10 +1,81 @@
-import { Suspense } from 'react'
-import { expect, test } from 'vitest'
-import { render, screen } from '@testing-library/react'
+import { StrictMode, Suspense } from 'react'
+import { expect, test, vi } from 'vitest'
+import { fireEvent, render, screen } from '@testing-library/react'
 
 import App from '.'
+import { runStorageProbe } from '../utils/image'
+
+vi.mock('../utils/image', async (importOriginal) => {
+  const actual = await importOriginal()
+  return {
+    ...actual,
+    cleanupStorageProbe: vi.fn().mockResolvedValue(undefined),
+    runStorageProbe: vi.fn(({ signal }) => new Promise((resolve, reject) => {
+      const passTimer = setTimeout(() => resolve({ writtenBytes: 1 }), 10)
+      signal.addEventListener('abort', () => {
+        clearTimeout(passTimer)
+        reject(new DOMException('Storage test canceled', 'AbortError'))
+      }, { once: true })
+    })),
+  }
+})
+
+function openStorageCheck() {
+  fireEvent.click(screen.getByRole('button', { name: 'Start' }))
+}
 
 test('renders without crashing', () => {
   render(<Suspense fallback="loading"><App /></Suspense>)
   expect(screen.getByText('flash.comma.ai')).toBeInTheDocument()
 })
+
+test('shows the storage check without private-browsing guidance', async () => {
+  render(<Suspense fallback="loading"><App /></Suspense>)
+  openStorageCheck()
+
+  expect(screen.getByRole('heading', { name: 'Checking available storage' })).toBeInTheDocument()
+  expect(screen.queryByText(/Do not use Incognito or InPrivate browsing/)).not.toBeInTheDocument()
+  expect(await screen.findByRole('heading', { name: 'Which device are you flashing?' })).toBeInTheDocument()
+})
+
+test('shows private-browsing guidance after a storage failure', async () => {
+  vi.mocked(runStorageProbe).mockRejectedValueOnce(new Error('Quota exceeded'))
+  render(<Suspense fallback="loading"><App /></Suspense>)
+  openStorageCheck()
+
+  expect(screen.queryByText(/Do not use Incognito or InPrivate browsing/)).not.toBeInTheDocument()
+  expect(await screen.findByRole('heading', { name: 'Storage check failed' })).toBeInTheDocument()
+  expect(screen.getByText(/Use a regular browser window/)).toBeInTheDocument()
+  expect(screen.getByText(/Free at least 6 GiB of space on this device/)).toBeInTheDocument()
+  expect(screen.getByText(/chrome:\/\/settings\/content\/siteData/)).toBeInTheDocument()
+  expect(screen.getByText(/Fully quit and reopen the browser/)).toBeInTheDocument()
+  expect(screen.getByText(/Android phone running Chrome, following the same steps above/)).toBeInTheDocument()
+})
+
+test('can skip the storage check', async () => {
+  render(<Suspense fallback="loading"><App /></Suspense>)
+  openStorageCheck()
+
+  fireEvent.click(screen.getByRole('button', { name: 'Skip storage check' }))
+  expect(await screen.findByRole('heading', { name: 'Which device are you flashing?' })).toBeInTheDocument()
+})
+
+test('Shift-clicking Skip storage check opens the failure guidance for testing', async () => {
+  render(<Suspense fallback="loading"><App /></Suspense>)
+  openStorageCheck()
+
+  fireEvent.click(screen.getByRole('button', { name: 'Skip storage check' }), { shiftKey: true })
+  expect(await screen.findByText(/Storage check failed/)).toBeInTheDocument()
+  expect(screen.getByRole('button', { name: 'Retry storage check' })).toBeInTheDocument()
+})
+
+test('does not cancel the storage pre-check during the Strict Mode effect cycle', async () => {
+  render(
+    <StrictMode>
+      <Suspense fallback="loading"><App /></Suspense>
+    </StrictMode>,
+  )
+  openStorageCheck()
+
+  expect(await screen.findByRole('heading', { name: 'Which device are you flashing?' })).toBeInTheDocument()
+})