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/drone_follow_icon.dart

@@ -13,7 +13,7 @@ class DroneFollowIcon extends StatelessWidget {
 
   @override
   Widget build(BuildContext context) {
-    // NavIconButton zwingt seinem Kind straffe 34x34-Constraints auf - ohne
+    // NavIconButton zwingt seinem Kind straffe 38x38-Constraints auf - ohne
     // Center/SizedBox wuerde CustomPaint sein size-Argument ignorieren und
     // auf die volle Knopfgroesse aufgeblasen (RenderCustomPaint hat anders
     // als Icon kein eingebautes Center, das die Constraints fuer sein Kind