Show all nav items on mobile instead of collapsing

The menu collapsed to a hamburger below 640px, which was roughly twice
the width the items actually need: measured with Segoe UI Semibold, the
five entries occupy 296px against 343px available at a 375px viewport.

Drops the toggle entirely and scales the column gap and font size with
clamp() instead, so there is no breakpoint at all. flex-wrap handles the
extreme case -- below ~330px the row wraps to two lines, which still
shows every item rather than hiding them.

Also removes the checkbox-and-label toggle hack, which announced itself
as a checkbox and carried no aria-expanded state.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

hugo.toml

@@ -48,10 +48,6 @@ buildFuture = false
     name = 'Archiv'
     pageRef = '/archiv'
     weight = 40
-  [[menus.main]]
-    name = 'Impressum'
-    pageRef = '/impressum'
-    weight = 50
 
 [outputs]
   home = ['html', 'rss', 'sitemap']