fix(math): add KaTeX CDN and fallback for radical SVG overflow

KaTeX renders square roots using a 400em-wide SVG that relies on CSS
`overflow: hidden` to clip to the correct length. Without the KaTeX
stylesheet, this SVG extends across the entire page.

Changes:
- Add KaTeX CDN stylesheet link to base.html for fonts and layout
- Add .katex-display local margin/overflow overrides
- Add .hide-tail svg max-width fallback for file:// CORS blocking

The CDN provides full KaTeX styling when accessible. The SVG constraint
serves as a fallback when CDN is blocked (e.g., local file:// access).

docs/static/style.css

@@ -127,6 +127,17 @@ a:hover {
   color: var(--accent-hover);
 }
 
+/* KaTeX display math - local overrides */
+.katex-display {
+  margin: 1rem 0;
+  overflow-x: auto;
+}
+
+/* Fallback: constrain radical SVG when CDN blocked */
+.hide-tail svg {
+  max-width: 100% !important;
+}
+
 /* Code */
 code {
   font-family: ui-monospace, "Cascadia Code", "Source Code Pro", Menlo, monospace;