Support metadata configuration for forbidden pages

[reasonjun]

Goals

1. Enable custom title and meta description for forbidden pages
2. Improve SEO optimization for 403 error pages
3. Provide better user experience with contextual information

Non-Goals

1. Changing the HTTP status code behavior (should remain 403)
2. Adding complex layout customization beyond metadata
3. Supporting dynamic metadata based on user authentication state

Background

Currently, the experimental forbidden pages feature doesn't support metadata configuration, which limits:

• SEO optimization for 403 pages
• Providing contextual information to users about access restrictions
• Maintaining consistent branding across error pages

Many applications need custom titles and descriptions for forbidden pages to explain why access is restricted and guide users to appropriate actions.

Proposal

Add support for metadata configuration in forbidden pages similar to other Next.js pages:

1. Support for metadata in forbidden.tsx
2. Enable setting custom title, description, other metadata
3. Maintain the 403 HTTP status while allowing metadata customization
4. Consider integration with existing Next.js metadata API patterns

Example usage:
export const metadata = {
title: "Access Restricted",
description: "This content requires special permissions"
}