Unify header pill/button height to 38px, matching the HTML prototype

Measured the prototype screenshot pixel-for-pixel: content pills take
up ~79% of the header's total height, matching its own CSS
(height:38px, topBarWrap padding:5px 10px -> 48px total). The Flutter
port had drifted to 34px for HeaderWindPill, MapSearchControls, and
NavIconButton, while FlightModePill was left at the original 38px -
so Fly mode looked inconsistent against itself, and both modes wasted
more header height as margin than the prototype does. Bringing all
four back to a shared 38px fixes both.

app/lib/ui/widgets/nav_icon_button.dart

@@ -28,8 +28,12 @@ class NavIconButton extends StatelessWidget {
     return GestureDetector(
       onTap: onTap,
       child: Container(
-        width: 34,
-        height: 34,
+        // 38 statt vorher 34, deckungsgleich mit HeaderWindPill/
+        // MapSearchControls/FlightModePill (Nutzerwunsch: einheitliche
+        // Kopfleisten-Hoehe wie im HTML-Demonstrator vermessen: #homeBtn/
+        // #fitBtn nutzen dort ebenfalls width/height:38px).
+        width: 38,
+        height: 38,
         decoration: BoxDecoration(
           shape: BoxShape.circle,
           color: Colors.black.withValues(alpha: 0.55),