Harden the email sanitizer's CSS handling

Rewrite mail CSS in place instead of cutting pieces out, so a strip can no
longer join text into a closing </style>, and escape < last. Decode escaped
letters before checking, parse url() properly and drop CSS that cannot be
parsed, and disable @import and image-set() in every spelling. The body
element's style goes through the same path.

Give <area> links the same target, rel and click handling as <a>, strip
<style> blocks from HTML quoted into the composer, and contain the editor's
layout as .message-body already is.

web/src/views/mail/MessageView.tsx

@@ -531,7 +531,7 @@ function HtmlBody({ html, bodyStyle, themed, forced, onShowImages, onFollowLink
   const onClick = useCallback(
     (ev: Event) => {
       const t = ev.target as HTMLElement;
-      const a = t.closest("a");
+      const a = t.closest("a, area");
       if (a) {
         const href = a.getAttribute("href") ?? "";
         if (href.startsWith("mailto:")) {