feat(binder): add render plots command for matplotlib figure gallery

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/

pyproject.toml

@@ -40,9 +40,10 @@ dependencies = [
     "pypandoc>=1.11",
     "pyyaml>=6.0",
 
-    # Data processing and validation
+    # Data processing and visualization
     "pandas>=2.0.0",
     "numpy>=1.24.0",
+    "matplotlib>=3.7.0",
     "jsonschema>=4.0.0",
 
     # Image processing