fix: update useFirestoreDocData return type to include undefined

[tyler-reitz]

Problem

useFirestoreDocData and useFirestoreDocDataOnce were declared to return ObservableStatus<T>, but docData() from rxfire returns Observable<T | undefined> because a Firestore document may not exist. This meant callers could access data.someProperty without any type-level warning, only to get a runtime error when data was undefined.

Fix

Update the return types to ObservableStatus<T | undefined>, which accurately reflects what the observable emits. The discriminated union in ObservableStatus already handles this correctly — status: 'loading' has data: undefined, status: 'success' has data: T | undefined for these hooks.

TypeScript now correctly flags:

const { data } = useFirestoreDocData(ref);
data.someProperty; // TS18048: 'data' is possibly 'undefined'

// Narrowing works as expected:
if (status === 'success' && data) {
  data.someProperty; // OK
}

Fixes #577

[gemini-code-assist]

Code Review

This pull request updates type definitions across several files. It replaces JSX.Element with React.ReactElement in auth.tsx, performance.tsx, and storage.tsx, and adds an explicit React.ReactElement return type in sdk.tsx. Additionally, it updates the return types of useFirestoreDocData and useFirestoreDocDataOnce in firestore.tsx to allow undefined values (ObservableStatus<T | undefined>). There are no review comments, and I have no feedback to provide.

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