feat(dashboard): add Next.js and TanStack installation snippets to tracking setup #1097

Description

On the website tracking setup page (/websites/[id]/settings/tracking), the "Install it yourself" section currently only has tabs for:

• Script tag
• React
• Vue
• Node.js

Since the majority of React users build with Next.js or TanStack Start / Router, having explicit tabs or snippets for these would improve onboarding clarity.

Motivation

• Next.js: Developers often wonder if <Databuddy /> works directly in Server Components (app/layout.tsx), how to use NEXT_PUBLIC_DATABUDDY_CLIENT_ID, and how to configure AI agent crawler tracking (@databuddy/sdk/agents in proxy.ts/middleware.ts).
• TanStack Router: Developers look for how to integrate tracking in __root.tsx with Vite environment variables.

Proposed Solution

• Add a dedicated Next.js tab showing app/layout.tsx integration with NEXT_PUBLIC_DATABUDDY_CLIENT_ID.
• Include a snippet or note for TanStack Router (__root.tsx).