docs: fix emulator setup example to prevent double-initialization

[tyler-reitz]

Problem

The emulator setup example in use.md called connect*Emulator() inside a React component body without any guard. This causes the following error on re-renders (React StrictMode, parent re-renders, Next.js page transitions):

FirebaseError: Firestore has already been started and its settings can no longer be changed. You can only modify settings before calling any other methods on a Firestore object.

This is the root cause behind all the workarounds in #459 (_settingsFrozen checks, window['_init'] flags, etc.) — users were working around bad docs.

Fix

Replace the broken component-level example with module-level initialization: create the app and SDK instances outside React, connect emulators there, then pass everything into providers via firebaseApp=. This guarantees connect*Emulator() only runs once.

Also adds a note explaining that firebaseConfig= on FirebaseAppProvider is a convenience shortcut that gives reactfire ownership of the app instance, making emulator setup unnecessarily difficult. Prefer firebaseApp= for anything non-trivial.

Fixes #459

[gemini-code-assist]

Code Review

This pull request updates the documentation in docs/use.md to demonstrate how to safely connect to the Firebase Local Emulator Suite, providing examples for initializing both outside of React and inside a component using a useRef guard. The reviewer pointed out that using useRef inside a component is still prone to double-initialization errors when the component unmounts and remounts, as the Firebase App instance is cached globally while the ref is recreated. They suggested using a module-level WeakSet to track initialized app instances instead.

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