fix: bound the service worker's navigation fetch with a timeout

[bluzername]

Fixes #274.

What was wrong

On iOS, launching the installed PWA from the Home Screen with no network
showed a black screen, then a blank white page, forever. The reporter
traced it exactly: sw.js's network-first fetch handler calls
fetch(e.request) with no timeout. A dead radio does not reject that
fetch quickly - it just never settles - so the .catch() a few lines
down, which falls back to the cached app shell, never gets a turn to run.
The app is stuck waiting on a request that will never finish.

The fix

Same one the reporter already tested and confirmed works for them:
fetch(e.request, { signal: AbortSignal.timeout(3000) }). After 3
seconds the fetch aborts, the existing .catch() fallback runs, and the
cached shell (or the matching cached asset) is served instead.

Test

sw.js lives in frontend/public/ and isn't itself under src/lib, so
there is no existing unit-test convention for it - and this isn't
something clicking through the app can reliably reproduce (it needs an
installed PWA and a truly dead network, not just devtools' "Offline"
checkbox, which rejects fetches immediately instead of hanging). Rather
than skip a test, I added
frontend/src/lib/sw-offline-launch.test.js, which loads the actual
production public/sw.js file into a sandboxed vm context (real
self/caches/fetch/AbortSignal stand-ins) and drives its real
fetch event listener directly:

• gives it a fetch that never resolves on its own (simulating a dead
  radio, not a quick rejection),
• asserts a timeout is actually wired to the request (this is what's
  missing on unmodified code - the test fails there with "expected 0 to
  be greater than 0" because AbortSignal.timeout is never called),
• asserts the response does not resolve on its own within a few
  microtask turns,
• fires the abort and asserts the response then resolves from the cache
  instead of hanging.

I confirmed this fails against unmodified sw.js (stashed the fix,
reran, got the RED failure above, restored the fix) and passes with it.

Verification

• cd frontend && npx vitest run src/lib/sw-offline-launch.test.js -
  1/1 pass.
• cd frontend && npx vitest run - full suite, 1573 tests (1 new), all
  pass (a few files hit a vitest worker-pool timeout under sandboxed
  parallelism and pass cleanly alone - unrelated to this change).

No new dependency, one line of production code changed
(frontend/public/sw.js), plus a new test file.

This account (bluzername) is not a collaborator on this repo, so I can't
merge - opening this for a maintainer to review.

🤖 Generated with Claude Code

[DuarteSantos8]

Thanks @bluzername, this fixes #274. I reworked it before merging: a 3 s timeout on every GET kept running while the body streamed, so on a slow link the 1.6 MB main bundle would have been cut mid-download. The timeout now applies only to navigations and is cleared once headers arrive, and a chunk or language pack is never cut off. iOS 15 keeps its offline start.

Released in v1.3.9: https://github.com/DuarteSantos8/openGym/releases/tag/v1.3.9