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

检查相关 issue #530 和 #334，然后阅读链接的 Firestore 序列化参考资料。探索在 React Server Components 中使用 Firestore，并确定哪些 reactfire hooks 与 RSC 兼容，哪些仅限客户端使用。完成标准是添加一个“Using reactfire with Next.js / RSC”文档部分，涵盖 'use client' 模式和 Firestore SSR 序列化方法。

由索引模型根据 Issue 内容生成。