Validate URL-like frontmatter fields in content schema

[plx]

Summary

Content schema fields that are semantically URLs are currently typed as plain strings, so invalid URLs in frontmatter are not rejected at validation time.

Affected file:

• src/content/config.ts

Examples:

• demoURL
• repoURL
• (and any other URL-like metadata fields)

Why This Matters

Schema validation should fail early on malformed metadata, preventing broken links from entering generated pages.

Proposed Strategy

• Update URL-like fields to use Zod URL validation (e.g., z.string().url().optional()).
• Decide if relative URLs are allowed; if yes, add a custom validator for absolute-or-relative policy.
• Update existing content entries if validation uncovers malformed values.

Done Criteria / Validation

• Invalid URL frontmatter fails astro check.
• Existing valid content still passes.
• npm run build and npm run validate:links pass.