types: initialData in ReactFireOptions<T> should accept snapshot types for snapshot hooks

## Summary

`ReactFireOptions` types `initialData` as `T`, which is correct for the data hooks (`useFirestoreDocData`, `useFirestoreCollectionData`, `useDatabaseObjectData`, etc.). For the raw snapshot hooks, the correct type is the snapshot wrapper, not the unwrapped data type:

| Hook | Correct `initialData` type |
|---|---|
| `useFirestoreDoc` | `DocumentSnapshot` |
| `useFirestoreDocOnce` | `DocumentSnapshot` |
| `useFirestoreCollection` | `QuerySnapshot` |

> **Corrected:** this table originally listed a fourth row, `useFirestoreCollectionOnce`. **No such export exists.** The raw snapshot hooks are the three above.

This mismatch is pre-existing and was silently masked by the `T | any` widening removed in #740. It is not a regression from that PR.

## Options

1. Split `ReactFireOptions` into separate types for data hooks vs snapshot hooks, each with the correct `initialData` type.
2. Make `initialData` generic at the hook level (e.g., `initialData?: TSnapshot`) independent of the data type `T`.

Either approach would be a TypeScript-only change with no runtime impact.

Tracked in #740 (known limitation section).

Start by locating ReactFireOptions and the raw snapshot hooks useFirestoreDoc, useFirestoreDocOnce, and useFirestoreCollection. Review the known limitation tracked in #740, then compare these types with the data hooks. Done means snapshot hooks accept their DocumentSnapshot or QuerySnapshot initialData types while data hooks retain T, with no runtime changes.

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