fix(ui): mobile nav items too low, text misaligned with borders

Root cause: the close button's base styles (float:right, margin with
1rem bottom) were never fully neutralized in the mobile override.
In a flex column container float is ignored, so the button consumed
~3.25rem as a flex row. Previous fix attempts added position:absolute
but left float and margin inherited, which could still affect layout
on iOS WebKit.

Fix: explicitly reset float:none and margin:0 alongside position:
absolute, using physical properties (top/right) for maximum browser
compatibility. Also remove first-child top border and zero out nav
link inline padding so text aligns flush with divider lines.

Author: danielnaab

src/design-system/components/flex-header/styles.css

@@ -367,8 +367,10 @@ flex-header {
   .flex-header__close-btn {
     display: block;
     position: absolute;
-    inset-block-start: 0.5rem;
-    inset-inline-end: 0.5rem;
+    top: 0.5rem;
+    right: 0.5rem;
+    float: none;
+    margin: 0;
   }
 
   .flex-header__nav-list {