follow mode button, zoom mode and abort behavior tuning

app/lib/ui/widgets/nav_icon_button.dart

@@ -6,18 +6,23 @@ import 'package:flutter/material.dart';
 class NavIconButton extends StatelessWidget {
   const NavIconButton({
     super.key,
-    required this.icon,
+    this.icon,
+    this.iconWidget,
     required this.onTap,
-    this.active = false,
-  });
+  }) : assert(
+          icon != null || iconWidget != null,
+          'Entweder icon oder iconWidget angeben.',
+        );
+
+  final IconData? icon;
+
+  /// Individuelles Icon-Widget statt eines Material-[IconData], z.B.
+  /// [DroneFollowIcon] fuer den Kartenfolge-Knopf (FlyMapControls). Hat
+  /// Vorrang vor [icon], falls beides gesetzt ist.
+  final Widget? iconWidget;
 
-  final IconData icon;
   final VoidCallback onTap;
 
-  /// Rosa Akzentrand (Doku 3.10: DroneMarkerIcon-Farbe) fuer Knoepfe mit
-  /// an/aus-Zustand, z.B. den Drohnen-Folgemodus (FlyMapControls).
-  final bool active;
-
   @override
   Widget build(BuildContext context) {
     return GestureDetector(
@@ -28,11 +33,8 @@ class NavIconButton extends StatelessWidget {
         decoration: BoxDecoration(
           shape: BoxShape.circle,
           color: Colors.black.withValues(alpha: 0.55),
-          border: active
-              ? Border.all(color: const Color(0xFFFF2E93), width: 2)
-              : null,
         ),
-        child: Icon(icon, color: Colors.white, size: 16),
+        child: iconWidget ?? Icon(icon, color: Colors.white, size: 16),
       ),
     );
   }