kobotoolbox / kpi / 37782804264

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fix(library): avoid null legacy router crash in library stores DEV-3061 (#7673)

### 💭 Notes

Fixes an intermittent `npm-test` failure in the Storybook test step,
e.g. [run
37465024804](https://github.com/kobotoolbox/kpi/actions/runs/37465024804)
(attempt 1, Firefox):

```
Features/LibraryUploadModal → UploadForm → play-test
can't access property "subscribe", _router_legacy__WEBPACK_IMPORTED_MODULE_7__.QB is null
```

Root cause: `LibraryUploadModal` imports `myLibraryStore`, whose
constructor (like `managedCollectionsStore`, `publicCollectionsStore`
and `singleCollectionStore`) schedules `setTimeout(() =>
router!.subscribe(...))`. The legacy `router` is only injected by
`router.tsx`, which Storybook never loads, so it stays `null` and the
callback throws on every Storybook run, in every browser. The test
runner only reports errors thrown while a play function is running, so
the test failed only when that timer happened to fire during the play
function.

Fix: use `router?.subscribe(...)` in the four library stores, as the
comment on `router` in `router/legacy.tsx` already recommends for
non-component callers ("may need to check null status").

- No change in the app: `router.tsx` injects the router when it loads,
before these timers fire.
- No change in the stories: the subscription only re-fetches data on
route changes, which never happen in Storybook. The other route checks
in the stores read `window.location.hash`, not the legacy `router`.
- Not a Storybook-side mock router: the timer is set when the story file
is imported, before any decorator runs, so injecting a router from a
decorator could still lose the race. A global router in
`.storybook/preview.tsx` would also clash with
`ProjectSettings.stories.tsx`, which resets the router to `null` when
its story unmounts.
- `router!.subscribe` in `ProjectSettings.tsx` and
`mainHeader.component.tsx` is left as is: those calls run when a
component mounts, not on import.

Out of scope: the more frequent `... (continued)

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