Point the fly-mode rubber band at home during Return-to-Home

iNAV freezes activeWaypointIndex at whatever mission waypoint was
active when RTH engaged instead of updating it to reflect the new
target - navigation.c derives NAV_Status.activeWpIndex unconditionally
from posControl.activeWaypointIndex, and none of the RTH state-entry
handlers touch that field. Drawing the guidance line to that stale
index would point at a waypoint the aircraft may have already passed.

Extracted the target selection into computeRubberBandTarget() (mission
stats.dart) so the RTH special-case and the abort-back-to-waypoint-mode
fallback are unit-tested rather than only living inline in the widget
build method - since it re-evaluates navMode on every frame, aborting
RTH switches the line back to the mission waypoint with no extra
transition logic needed.

app/test/domain/mission_stats_test.dart

@@ -1,6 +1,7 @@
 import 'package:dmc_app/domain/mission/mission_stats.dart';
 import 'package:dmc_app/domain/waypoint/flat_waypoint_list.dart';
 import 'package:flutter_test/flutter_test.dart';
+import 'package:latlong2/latlong.dart';
 
 Waypoint _wp(double lat, double lon, {double speed = 15}) =>
     Waypoint(lat: lat, lon: lon, altitudeM: 60, speedMs: speed, catchRadiusM: 60);
@@ -110,4 +111,80 @@ void main() {
       expect(status!.etaToActiveWaypointSec, isNull);
     });
   });
+
+  group('computeRubberBandTarget', () {
+    final waypoints = [_wp(0, 0), _wp(0, 0.01), _wp(0, 0.02)];
+    const home = LatLng(1, 1);
+
+    test('zeigt normalerweise auf den aktiven Missionswegpunkt', () {
+      final target = computeRubberBandTarget(
+        navMode: 3, // Waypoint mission
+        activeWaypointIndex: 1,
+        waypoints: waypoints,
+        homePoint: home,
+      );
+      expect(target, const LatLng(0, 0.01));
+    });
+
+    test('zeigt bei Return-to-Home (navMode 2) auf den Homepoint statt auf '
+        'den (ggf. laengst passierten) eingefrorenen Missionswegpunkt', () {
+      final target = computeRubberBandTarget(
+        navMode: navModeReturnToHome,
+        activeWaypointIndex: 1,
+        waypoints: waypoints,
+        homePoint: home,
+      );
+      expect(target, home);
+    });
+
+    test('liefert null bei RTH ohne bekannten Homepoint statt eines '
+        'erfundenen Ziels', () {
+      final target = computeRubberBandTarget(
+        navMode: navModeReturnToHome,
+        activeWaypointIndex: 1,
+        waypoints: waypoints,
+        homePoint: null,
+      );
+      expect(target, isNull);
+    });
+
+    test('schaltet nach Abbruch von RTH (navMode wieder 3) automatisch '
+        'zurueck auf den Missionswegpunkt', () {
+      const rth = 2;
+      final duringRth = computeRubberBandTarget(
+        navMode: rth,
+        activeWaypointIndex: 1,
+        waypoints: waypoints,
+        homePoint: home,
+      );
+      final afterAbort = computeRubberBandTarget(
+        navMode: 3,
+        activeWaypointIndex: 1,
+        waypoints: waypoints,
+        homePoint: home,
+      );
+      expect(duringRth, home);
+      expect(afterAbort, const LatLng(0, 0.01));
+    });
+
+    test('activeWaypointIndex null oder ausserhalb der Liste ergibt null '
+        '(ausserhalb RTH)', () {
+      expect(
+        computeRubberBandTarget(
+          navMode: 3,
+          activeWaypointIndex: null,
+          waypoints: waypoints,
+        ),
+        isNull,
+      );
+      expect(
+        computeRubberBandTarget(
+          navMode: 3,
+          activeWaypointIndex: 3,
+          waypoints: waypoints,
+        ),
+        isNull,
+      );
+    });
+  });
 }