Skill: Next.js + Electric integration

## Context

Next.js is a common framework pairing with Electric. The proxy pattern differs from TanStack Start (API routes vs server functions), and there are Next.js-specific considerations around App Router vs Pages Router, server components, and middleware.

## Scope

Create an `electric-nextjs` skill covering:

1. **Proxy route implementation** — App Router route handler (`app/api/shapes/route.ts`) vs Pages Router API route
2. **Server-side shape definition** — Where to inject ELECTRIC_SECRET, user-scoped WHERE clauses
3. **Electric protocol param forwarding** — Using ELECTRIC_PROTOCOL_QUERY_PARAMS
4. **Header forwarding** — Preserving electric-offset, electric-handle, electric-schema
5. **Middleware patterns** — Auth validation before proxy forwarding
6. **TanStack DB integration** — Collections with Next.js-specific proxy URL patterns
7. **Common gotchas** — CORS, Vercel serverless timeouts with SSE, edge runtime limitations

## Source

Identified during domain discovery skill test run. Maintainer confirmed Next.js warrants standalone skill due to integration complexity.