Fix schema.org types and social handles

Current State

• Authors.svelte: every author <li> is itemtype="https://schema.org/Organization"; the Author type even has an org flag that is not used to switch between Person and Organization
• In listings the itemprop="author" items sit inside <ul> with no enclosing Article scope
• CallToAction.svelte: itemtype="https://schema.org/Offer" with a hidden seller Organization whose URL is https://sveleton.com/ — a domain the project does not own
• Meta.svelte emits twitter:site as @Sveleton (derived from PROJECT_NAME) and a keywords meta tag no search engine reads

Tasks

• Use Person unless author.org is true
• Only emit itemprop="author" inside an Article scope
• Drop the Offer markup from the CTA (or make it opt-in with real seller data)
• Make twitter:site a separate optional PROJECT_TWITTER constant; remove keywords
• Validate a feature page with the Schema Markup Validator