Integrate figure rendering into the binder CLI so plots can be previewed
without a full Quarto build. Extracts Python code blocks with fig-* labels
from QMD files, renders them to PNG, and outputs a browsable gallery at
_output/plots/<chapter>/. Also fixes the package import chain so `binder`
works correctly as an installed entry point.
- Add book/cli/commands/render.py with RenderCommand class
- Wire into main.py with help table entry and command dispatch
- Add matplotlib>=3.7.0 to pyproject.toml dependencies
- Add book/quarto/_output/ to .gitignore
- Archive standalone render_figures.py to _archive/
