feat: add image support to posts, write spool & ferrotype fragments

- Add optional images array to rip.nate.post lexicon (blobs on PDS)
- Upload script scans markdown for local image refs, uploads as blobs,
  rewrites paths to blob URLs, supports --date and --dry-run flags
- Add prose img styling to detail page
- Spool post (#56) with 3 TUI screenshots
- Ferrotype post (#57) with detailed TypeScript examples