Update to Next js APP router.

[rvadherhztl03]

Summary

This PR migrates the storefront from the Next.js Pages Router to the App Router and updates the project to support newer Next.js, React, and related dependencies.

Changes Made

• Migrated page routes from pages/ to the new app/ directory structure.
• Added App Router layouts, including global metadata, favicon configuration, OrderCloud provider setup, and shared layout wrapping.
• Updated product listing, product detail, cart, checkout, login, confirmation, and home pages for App Router compatibility.
• Migrated checkout API routes into App Router route handlers under app/api.
• Added webhook helper support for adapting OrderCloud webhook-style handlers to App Router Request / Response APIs.
• Updated routing logic to use next/navigation instead of next/router.
• Marked client-side components with "use client" where hooks, Redux, router APIs, or browser behavior are required.
• Updated Next.js, React, React DOM, React Redux, TypeScript, Node, and React type dependencies.
• Adjusted TypeScript and ESLint configuration for the newer Next.js setup.
• Cleaned up several TypeScript issues and improved typings around layout children, checkout submission, payments, and product detail handling.

Testing

• Verified migration structure and route changes against the current branch diff.
• Please run the app locally and test product listing, product detail, cart, checkout, login, and order confirmation flows before merging.