fix: align Node.js requirements and validate metadata URLs

[Copilot]

Description

Addresses review feedback on PR #3: Node.js version constraints were inconsistent across docs/CI/package.json, openGraph.url was hardcoded to GitHub instead of deriving from deployment URL, and NEXT_PUBLIC_SITE_URL lacked validation.

Changes:

• Metadata URL handling: Removed hardcoded openGraph.url—now derives from metadataBase. Added getMetadataBaseUrl() to validate NEXT_PUBLIC_SITE_URL has protocol, fallback to localhost:3000 if missing/malformed.

• Node.js version alignment: Updated badge (18+ → 18.18.0+), added engines.node: ">=18.18.0" to package.json, changed CI matrix (18.x → 18.18.x) to match Next.js 15 requirement.

function getMetadataBaseUrl(): string {
  const siteUrl = process.env.NEXT_PUBLIC_SITE_URL;
  if (!siteUrl) return 'http://localhost:3000';
  if (!/^https?:\/\//i.test(siteUrl)) {
    console.warn(`NEXT_PUBLIC_SITE_URL ('${siteUrl}') missing protocol. Using default: http://localhost:3000`);
    return 'http://localhost:3000';
  }
  return siteUrl;
}

Type of Change

• Bug fix
• New feature
• Documentation
• Refactoring

Testing

• Lint and build passed
• CodeQL security scan clean

Checklist

• Code follows project style guidelines
• No debug code left in
• Tested locally
• Updated README if needed

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