Pin emails to one line, hide single-entry TOCs

Two small quality-of-life fixes noticed during a mobile review.

## Email addresses never wrap

Auto-linked `<a href="mailto:...">` elements now carry
`white-space: nowrap` via a global CSS rule. `sw@wintermeyer-consulting.de`
is short enough that it fits on any viewport, so this just prevents the
browser from breaking an address between the `@` and the domain (or
worse, mid-TLD) when a paragraph happens to wrap at that point. Applies
site-wide (header, body, footer, Kontakt button) since every email
occurrence already ships as an `a[href^="mailto:"]` in the output.

## TOC with a single entry is removed

Antora's default client-side TOC (`aside.toc`) removes itself when the
page has no matching headings. It happily renders a one-item list
though — just the one section title, which looks like dead weight on
pages like `recipes/index` that only have `h1 + h2`. Tightened the
check in `site.js` from `if (!s.length)` to `if (s.length < 2)`, so
the aside (and the embedded mobile copy) is dropped unless the page
has at least two anchors worth linking to.

Verified in a 390px-wide iframe harness: `recipes/index.html` now has
no TOC, `ash/index.html` (two `h2`s) keeps its TOC, and every mailto
link reports `whiteSpace: nowrap` with no increased page scrollWidth.

Author: wintermeyer

ui-bundle/src/css/site.css

@@ -272,6 +272,13 @@ main.article > .content {
   @apply text-[#65a30d] underline decoration-[#65a30d]/40 underline-offset-[3px] hover:decoration-[#65a30d] dark:text-[#c8f55a] dark:decoration-[#c8f55a]/40 dark:hover:decoration-[#c8f55a];
 }
 
+/* Email addresses never wrap mid-address, even if they push the line
+   wrap point out. Applies globally (header, body, footer) so a mailto
+   link always renders as one uninterrupted token. */
+a[href^="mailto:"] {
+  white-space: nowrap;
+}
+
 .doc a.anchor {
   @apply hidden;
 }