Spike: Next.js / React Server Components usage guide

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 #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

Prüfe die verwandten Issues #530 und #334 und lies anschließend die verlinkte Referenz zur Firestore-Serialisierung. Untersuche die Verwendung von Firestore mit React Server Components und ermittle, welche reactfire-Hooks RSC-kompatibel und welche ausschließlich für den Client bestimmt sind. Als abgeschlossen gilt die Ergänzung eines Dokumentationsabschnitts „Using reactfire with Next.js / RSC“, der das Muster 'use client' und den Ansatz zur Firestore-SSR-Serialisierung abdeckt.

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