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/src/app.css

@@ -517,6 +517,8 @@ input::placeholder { color: var(--muted); }
   margin: 0 0 1em;
 }
 .md pre code { background: none; border: 0; padding: 0; font-size: inherit; }
+/* A long equation scrolls on its own rather than widening the page. */
+.md .katex-display { overflow-x: auto; overflow-y: hidden; padding: 2px 0; }
 .md hr { border: 0; border-top: 1px solid var(--overlay); margin: 2.4em 0; }
 .md img, .md video {
   max-width: 100%;