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/package.json

@@ -27,6 +27,7 @@
     "@lezer/highlight": "^1.2.1",
     "@replit/codemirror-vim": "^6.2.1",
     "dompurify": "^3.2.4",
+    "katex": "^0.19.0",
     "marked": "^15.0.6"
   }
 }