Fix column overflow and render math with KaTeX

- minmax(0,1fr)+min-width:0 and pre-wrap so a long fenced line no longer blows out the grid
- build-time KaTeX via @vscode/markdown-it-katex; ship katex CSS+fonts and link in layouts

.eleventy.js

@@ -1,5 +1,6 @@
 const Image = require("@11ty/eleventy-img");
 const path = require("path");
+const markdownItKatex = require("@vscode/markdown-it-katex").default;
 
 function imageShortcode(src, alt, sizes = "100vw") {
   const inputPath = path.join("img", src);
@@ -54,6 +55,17 @@ module.exports = function (eleventyConfig) {
   eleventyConfig.addPassthroughCopy("img");
   eleventyConfig.addPassthroughCopy("favicon.svg");
 
+  // KaTeX: render $$...$$ at build time into static markup. Ship katex.min.css
+  // plus its fonts/ as a sibling so the stylesheet's url(fonts/...) resolves.
+  eleventyConfig.addPassthroughCopy({
+    "node_modules/katex/dist/katex.min.css": "css/katex.min.css",
+    "node_modules/katex/dist/fonts": "css/fonts",
+  });
+
+  eleventyConfig.amendLibrary("md", (md) => {
+    md.use(markdownItKatex, { throwOnError: false, errorColor: "#cc0000" });
+  });
+
   eleventyConfig.addShortcode("image", imageShortcode);
   eleventyConfig.addShortcode("heroImage", heroImageShortcode);