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>

layouts/_partials/header.html

@@ -22,10 +22,6 @@
 
   <nav class="site-nav" aria-label="Hauptnavigation">
     <div class="container">
-      <input type="checkbox" id="nav-toggle" class="nav-toggle">
-      <label for="nav-toggle" class="nav-toggle-label">
-        <span aria-hidden="true">☰</span> Menü
-      </label>
       <ul class="nav-list">
         <li><a href="{{ "/" | relURL }}"{{ if .IsHome }} aria-current="page"{{ end }}>Start</a></li>
         {{ range site.Menus.main }}