markdown: math set by KaTeX, fetched only when a page has some

$x^2$ inline, $$…$$ for display, and a ```math block, the way GitHub
writes them, in documents, shared pages and .md files alike. KaTeX loads
on first use; until then the math shows as its source. Only the woff2
fonts are bundled.

web/vite.config.ts

@@ -1,8 +1,22 @@
 import { defineConfig } from 'vite'
 import { svelte } from '@sveltejs/vite-plugin-svelte'
+import type { Plugin } from 'vite'
+
+/*
+ * KaTeX lists each font as woff2, woff and ttf. Every browser the app runs in
+ * takes woff2, and the other two would be another megabyte in the binary.
+ */
+const katexWoff2Only: Plugin = {
+  name: 'katex-woff2-only',
+  enforce: 'pre',
+  transform(code, id) {
+    if (!/katex(\.min)?\.css$/.test(id)) return
+    return code.replace(/,\s*url\([^)]*\.(?:woff|ttf)\)\s*format\("(?:woff|truetype)"\)/g, '')
+  },
+}
 
 export default defineConfig({
-  plugins: [svelte()],
+  plugins: [svelte(), katexWoff2Only],
   build: { target: 'es2022', chunkSizeWarningLimit: 900 },
   server: {
     port: 4243,