Spike: Next.js / React Server Components usage guide

[tyler-reitz]

reactfire works with Next.js but the full SSR/RSC story needs exploration before we can write authoritative guidance. Known pieces:

Solved (client-side patterns):

• Context providers need 'use client' for App Router
• Browser-only SDKs (Analytics, etc.) also need ssr: false via next/dynamic to prevent SSR initialization — see Firebase getAnalytics() throws Error: Component analytics has not been registered yet #530

Needs exploration:

• How to use Firestore with React Server Components, including serialization constraints (Firestore objects can't cross the RSC boundary as-is)
• Reference: https://firebase.blog/posts/2026/06/firestore-serialization-react
• General guidance for which reactfire hooks are RSC-compatible vs. client-only

Outcome: a "Using reactfire with Next.js / RSC" section in docs covering both the simple 'use client' pattern and the Firestore SSR serialization approach.

Related: #530, #334