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/lib/ui/screens/fly/fly_screen.dart

@@ -293,23 +293,25 @@ class _FlyScreenState extends ConsumerState<FlyScreen> {
                 child: const DroneMarkerIcon(),
               );
 
-    // Gestrichelte Linie von der Drohne zum aktiven Wegpunkt (Doku 3.4:
+    // Gestrichelte Linie von der Drohne zum aktuellen Flugziel (Doku 3.4:
     // TelemetryFrame.activeWaypointIndex aus MSP_NAV_STATUS) - derselbe
     // Darstellungsstil wie das Reticle-Gummiband im Plan-Modus (MissionMap.
-    // rubberBandLine), hier aber Drohne -> naechster Wegpunkt statt letzter
-    // Wegpunkt -> Kartenmitte.
-    List<LatLng>? rubberBand;
+    // rubberBandLine), hier aber Drohne -> Flugziel statt letzter Wegpunkt
+    // -> Kartenmitte. Das Ziel selbst (Missionswegpunkt oder bei Return-to-
+    // Home der Homepoint) kommt aus computeRubberBandTarget, siehe dortige
+    // Doku fuer den RTH-Sonderfall.
     final activeWpIndex = telemetry?.activeWaypointIndex;
-    if (telemetry != null &&
-        telemetry.hasFix &&
-        activeWpIndex != null &&
-        activeWpIndex >= 0 &&
-        activeWpIndex < waypoints.length) {
-      final target = waypoints[activeWpIndex];
-      rubberBand = [
-        LatLng(telemetry.lat, telemetry.lon),
-        LatLng(target.lat, target.lon),
-      ];
+    List<LatLng>? rubberBand;
+    if (telemetry != null && telemetry.hasFix) {
+      final target = computeRubberBandTarget(
+        navMode: telemetry.navMode,
+        activeWaypointIndex: activeWpIndex,
+        waypoints: waypoints,
+        homePoint: homePoint == null ? null : LatLng(homePoint.lat, homePoint.lon),
+      );
+      if (target != null) {
+        rubberBand = [LatLng(telemetry.lat, telemetry.lon), target];
+      }
     }
 
     // Live-Missionsstatus fuer die Details-Pille (Doku: "aktiver Wegpunkt