fix: cleanup event-emitter subscriptions in provider hooks

[Zaimwa9]

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Changes

Closes #390
Hooks were rendering at render time instead than in an effect and the cleanup paths did not remove all registered listeners

• useFlagsmithLoading and useFlags: subscribe inside useEffect with emitter.on's returned unsubscribe as cleanup. Removes render-time events.on and multi subscription

How did you test this code?

Branch	Active listeners after 1000 SSR renders
main (pre-fix)	2000 (2 leaked per render: one from useFlagsmithLoading, one from useFlags)
fix/react-subscription-leak	0

[talissoncosta]

Looks good, just one question. Not blocking