fix ascii art rendering: preserve whitespace in code blocks

Author: jf---

docs/examples/02_curved_slicing.md

@@ -21,7 +21,7 @@ Traditional planar slicing creates horizontal layers. For curved surfaces like s
 
 **Curved slicing** generates toolpaths that follow the surface curvature:
 
-```
+```text
 Planar slicing:          Curved slicing:
     ___________              _________
    |___________|            /         \

docs/stylesheets/extra.css

@@ -11,6 +11,12 @@
   font-size: 0.85em;
 }
 
+/* Preserve whitespace in code blocks for ASCII art */
+.md-typeset pre code {
+  white-space: pre;
+  font-family: 'JetBrains Mono', 'Courier New', monospace;
+}
+
 /* API docs styling */
 .doc-heading {
   font-weight: 600;