Featured image picker and og:image for posts

Created by: jedmund

Summary

• Add ImagePicker to PostMetadataForm for selecting a featured image
• Wire featuredImage through post editor state, dirty tracking, and save/load
• Use featuredImage for og:image and JSON-LD, falling back to first attachment

Test plan

• Open post editor metadata tab → select a featured image via picker → save → reload → verify it persists
• Publish a post with a featured image → view page source → verify og:image uses the featured image URL
• Publish a post without a featured image but with attachments → verify og:image falls back to first attachment