fix(sitemap): skip external slugs and guard invalid dates

[qianzhu18]

Summary

Fixes sitemap URL generation when menu/page slugs are external links, and guards invalid publishDay values to avoid invalid date output.

Closes #3668

Problem

• External links in slug were appended to site domain, producing invalid URLs like:
  • https://example.com/https://external.com/page
• Invalid dates could leak Invalid Date behavior into sitemap generation paths.

Root Cause

• Sitemap builders treated all slugs as relative paths.
• Date formatting called toISOString() without invalid-date fallback.

Changes

• Added lib/sitemap-utils.js for shared sitemap normalization:
  • base URL normalization
  • locale normalization
  • URL building with external-host filtering
  • safe date formatting fallback
• Updated pages/sitemap.xml.js to use normalized URL/date helpers.
• Updated lib/sitemap.xml.js with the same guards.
• Added tests:
  • __tests__/lib/sitemap-utils.test.js
  • __tests__/lib/sitemap.xml.test.js

Verification

• npm test -- __tests__/lib/sitemap-utils.test.js __tests__/lib/sitemap.xml.test.js
• npm run lint -- --file pages/sitemap.xml.js --file lib/sitemap.xml.js --file lib/sitemap-utils.js --file __tests__/lib/sitemap-utils.test.js --file __tests__/lib/sitemap.xml.test.js

Risk & Compatibility

• Low risk: changes are scoped to sitemap generation only.
• Behavior change: external absolute URLs on different host are excluded from sitemap (expected for site-local sitemap entries).

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[qianzhu18]

Heads-up: Vercel checks are failing with "Authorization required to deploy" from my fork environment. I verified this change locally with targeted lint/tests listed in the PR description.