Replace Vercel favicon with Arcade logo in Next.js template

Removes the default Next.js favicon.ico and icon.png (Vercel logo) and
replaces them with icon.svg using the Arcade logo. The langchain template
already had the correct Arcade favicon.svg.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: teallarson