Because:
* Sign-in moves to /pair in the router, so useFxAStatus kept the
signed-out answer from the sign-in page and reported it as current.
* /pair's own 1 s check waits behind Firefox's oauth_login token
exchange, so a slow exchange left /pair acting on that stale answer.
* That could bounce a just-signed-in Sync user to cached sign-in, and
likely explains the v1 screen seen on a v2-capable Firefox.
This commit:
* Reports fxaStatusState as 'pending' during render when isPairing
changes, before pages' effects can act on the old answer.
* Adds a hook test that a page sees 'pending' on a move to /pair.
Closes #FXA-14762