fix: position overdue nav-badge absolutely to avoid flex layout distortion (closes #56)

The nav-badge was appended as an in-flow flex child, breaking nav-item
layout: on mobile (column flex) it appeared below the label, on desktop
sidebar (row flex + justify-content:center) it was pushed far right via
margin-left:auto. Fix positions it absolutely within the nav-item and
uses DOM API instead of insertAdjacentHTML per project convention.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

public/styles/layout.css

@@ -193,6 +193,14 @@
   min-height: var(--target-lg);
   min-width: var(--target-lg);
   text-decoration: none;
+  position: relative;
+}
+
+.nav-item .nav-badge {
+  position: absolute;
+  top: var(--space-1);
+  right: var(--space-1);
+  margin-left: 0;
 }
 
 .nav-item:active {