Hover cards for domains and people, and defaults in option tooltips

- A domain's name in any list opens a card on hover: whether it's taking
  mail, how many people it has, which of its key records (mail routing,
  SPF, DKIM, DMARC) are live in public DNS, and whether DNS, signing keys
  and certificates are automatic.
- A person's shows their name, storage against their quota, role, groups
  and when they joined. The server keeps no last sign-in on the account,
  so the card doesn't claim one.
- Cards load on open and are cached for a minute.
- Option tooltips end with the option's default, and an option set away
  from its default gets a small "changed" mark.

src/help/HelpTip.tsx

@@ -16,9 +16,20 @@ import { manualUrl } from './manual';
  * focus, and a way into the manual once there is one. `id` is the option's
  * stable help id, the key the manual links hang on.
  */
-export function HelpTip({ id, text, className }: { id?: string; text?: string | null; className?: string }) {
+export function HelpTip({
+  id,
+  text,
+  footnote,
+  className,
+}: {
+  id?: string;
+  text?: string | null;
+  /** A short line under the text, like the option's default. */
+  footnote?: string | null;
+  className?: string;
+}) {
   const { t } = useTranslation();
-  if (!text) return null;
+  if (!text && !footnote) return null;
   const more = id ? manualUrl(id) : null;
   return (
     <TooltipProvider delayDuration={150}>
@@ -42,8 +53,9 @@ export function HelpTip({ id, text, className }: { id?: string; text?: string |
           className="max-w-xs space-y-1.5 border bg-popover px-3 py-2 text-xs leading-relaxed text-popover-foreground shadow-soft"
         >
           <div className="[&_code]:rounded [&_code]:bg-muted [&_code]:px-1 [&_p]:m-0">
-            <ReactMarkdown>{text.replace(/\\n/g, '\n')}</ReactMarkdown>
+            {text && <ReactMarkdown>{text.replace(/\\n/g, '\n')}</ReactMarkdown>}
           </div>
+          {footnote && <p className="text-muted-foreground">{footnote}</p>}
           {more && (
             <a
               href={more}