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/domain/mission/mission_stats.dart

@@ -98,3 +98,41 @@ FlyMissionStatus? computeFlyMissionStatus({
     etaToActiveWaypointSec: eta,
   );
 }
+
+/// `MW_GPS_MODE_RTH` (siehe MspNavMode._modeNames in msp_telemetry_codec.dart).
+const navModeReturnToHome = 2;
+
+/// Ziel des Gummiband-Pfeils von der Drohne zum aktuellen Flugziel im
+/// Fly-Modus (Doku: MissionMap.rubberBandLine) - normalerweise der aktive
+/// Missionswegpunkt, waehrend Return-to-Home ([navModeReturnToHome])
+/// stattdessen [homePoint].
+///
+/// iNAV friert `activeWaypointIndex` beim RTH-Eintritt auf dem zuletzt
+/// aktiven Missionswegpunkt ein, statt ihn auf den Homepoint zu setzen
+/// (`navigation.c`: `NAV_Status.activeWpIndex` wird unbedingt aus
+/// `posControl.activeWaypointIndex` abgeleitet, keiner der
+/// `navOnEnteringState_NAV_STATE_RTH_*`-Handler fasst diesen Wert an) - ein
+/// Gummiband zu diesem (ggf. laengst passierten) Wegpunkt waere im RTH-Fall
+/// irrefuehrend. Da [navMode] bei jedem neuen Telemetrie-Frame neu geprueft
+/// wird, schaltet ein Abbruch von RTH zurueck in den Wegpunktmodus das
+/// Ergebnis von selbst wieder auf den Missionswegpunkt zurueck.
+///
+/// Liefert null, wenn kein sinnvolles Ziel bekannt ist (RTH ohne bereits
+/// abgefragten Homepoint, oder [activeWaypointIndex] ausserhalb der Liste).
+LatLng? computeRubberBandTarget({
+  required int navMode,
+  required int? activeWaypointIndex,
+  required List<Waypoint> waypoints,
+  LatLng? homePoint,
+}) {
+  if (navMode == navModeReturnToHome) {
+    return homePoint;
+  }
+  if (activeWaypointIndex == null ||
+      activeWaypointIndex < 0 ||
+      activeWaypointIndex >= waypoints.length) {
+    return null;
+  }
+  final target = waypoints[activeWaypointIndex];
+  return LatLng(target.lat, target.lon);
+}