fix(site): PDF build improvements and community section

- Add npx pre-flight check for Mermaid CLI in PDF build
- Configure mermaid_cmd to use npx (no global install needed)
- Add Unicode box-drawing fallbacks for LaTeX
- Add Community section to PDF table of contents

tinytorch/site/Makefile

@@ -29,6 +29,9 @@ serve: clean html
 pdf:
 	@echo "🔥 Building Tiny🔥Torch PDF via XeLaTeX..."
 	@echo ""
+	@# PDF build needs Mermaid CLI (for diagrams). Config uses npx; Node.js must be installed.
+	@command -v npx >/dev/null 2>&1 || { echo "❌ Error: npx not found. PDF build requires Node.js (for Mermaid diagrams). Install from https://nodejs.org/ or run: brew install node"; exit 1; }
+	@echo "✅ Mermaid CLI (via npx) available"
 	@# Step 1: Generate LaTeX (matches CI workflow)
 	@echo "📚 Generating LaTeX files..."
 	jupyter-book build . --builder latex --config _config_pdf.yml --toc _toc_pdf.yml

tinytorch/site/_config_pdf.yml

@@ -51,6 +51,8 @@ sphinx:
     - sphinxcontrib.mermaid
   config:
     mermaid_version: "10.6.1"
+    # Use npx so PDF build works without global mermaid-cli (requires Node.js)
+    mermaid_cmd: "npx -y @mermaid-js/mermaid-cli"
     # Render mermaid as PDF for LaTeX
     # --pdfFit scales PDF to fit the diagram (not full page)
     # --scale 1.0 keeps diagrams at natural size (1.5 was too large for tall diagrams)
@@ -172,6 +174,13 @@ sphinx:
         \newunicodechar{ω}{$\omega$}
         \newunicodechar{✓}{\checkmark}
         \newunicodechar{✗}{$\times$}
+        % Box-drawing (terminal/code art) - TeX Gyre Cursor has no Unicode box glyphs
+        \newunicodechar{═}{\texttt{=}}
+        \newunicodechar{║}{\texttt{|}}
+        \newunicodechar{╔}{\texttt{+}}
+        \newunicodechar{╗}{\texttt{+}}
+        \newunicodechar{╚}{\texttt{+}}
+        \newunicodechar{╝}{\texttt{+}}
 
         % Syntax-highlighted code blocks with refined colors
         \usepackage{listings}

tinytorch/site/_toc_pdf.yml

@@ -121,6 +121,13 @@ parts:
   - file: conclusion
     title: "You Built Something Real"
 
+# Community (referenced from TITO overview)
+- caption: Community
+  numbered: false
+  chapters:
+  - file: community
+    title: "Community Guide"
+
 # Appendix
 - caption: Appendix
   numbered: false