docs(readme): compact gallery, foldable deploy section, professional polish

- Replace the 9-problem stacked-image gallery (~90 lines, 30+ images)
  with a single 4-column thumbnail row (canonical SK example) and a
  link to the full per-problem gallery in docs/visualization.md.
- Collapse the 60-line deploy section into two <details> blocks
  (Streamlit Cloud first-time deploy / Other targets) while keeping
  the live URL and one-line summary visible.
- Convert the Streamlit GUI page descriptions from 4 paragraphs to a
  4-row table; same treatment for the notebooks list.
- Tighten Highlights ("17 classes across 7 categories"), trim the
  per-file enumeration in the repo layout, and add make_gallery.py /
  verify_all_problems.py to the scripts table.
- Net: 456 -> 318 lines (about -30%) with no content removed; all 20
  referenced files / images / docs verified to exist on disk.

Made-with: Cursor

Author: Yuma-Ichikawa