Move module-scope Emitter in react.tsx into the Provider

[Zaimwa9]

react.tsx instantiates a single Emitter at module scope, which means all <FlagsmithProvider> trees in a process share the same event bus.

On long-lived Node hosts (Fluid Compute, serverful SSR) this is a cross-request coupling hazard setups it causes unrelated providers to notify each other's hooks (safe today thanks to value comparisons).

Objective
Move the emitter to a useRef in FlagsmithProvider and pass it via context alongside flagsmith; hooks read it from context.

Surfaced by #390

[kekius-maximus45]

@Zaimwa9
Hi, I’d like to work on this. I’ll move the module-scoped Emitter into FlagsmithProvider via useRef, pass it through context with the flagsmith instance, and update the React hooks to consume the provider-local emitter. I’ll also run typecheck/tests and add coverage if there’s an existing React test pattern for this.