Add Next.js App Router example

[satoshai-dev]

Summary

Add a Next.js App Router example app alongside the existing examples/vite-react/ to demonstrate kit usage in the most common React framework setup.

Location

examples/nextjs/

Should cover

• StacksWalletProvider placement in a root layout (app/layout.tsx) with "use client" wrapper
• Connect/disconnect flow
• Display connected address + BNS name
• Wallet selection UI using useWallets
• A contract write example (basic, untyped is fine for now — typed example can be added after Typed useWriteContract with ABI inference #35)
• Minimal styling, focus on clarity not aesthetics

Stack

• Next.js 15 (App Router)
• @satoshai/kit via workspace:*
• @stacks/transactions as peer dep
• TypeScript

Notes

• Keep it minimal — this is a reference, not a template
• Should work with pnpm dev from the example directory
• Add to pnpm-workspace.yaml under examples/* (already covered by glob)