Payment completed and Flux was credited, but returning from Stripe
showed `ERR_FAILED`. Edge reported that the service worker replayed a
redirected response for a navigation request. Cloudflare redirects
`/index.html` to `/`.
Precache the canonical root URL and use it as the navigation fallback.
The new cache key also avoids reusing the old redirected app shell.
Validation: generated a service worker with the installed Workbox
version and verified both its precache entry and navigation handler use
`./`; ESLint passed. Production payment credited 500 Flux, and bypassing
the service worker restored the page. Normal-cache production
verification follows deployment.