Show distance to home as a fourth field in the fly-mode details pill

Adds computeDistanceToHomeM() (mission_stats.dart) alongside the
existing rubber-band-target helper, wired through MissionFooterBar ->
BottomStatsBar the same way flyMissionStatus already is. Only appears
once a home point is known, as the last field after waypoint/distance/
duration.

Generalized HomePointIcon to take an optional strokeColor: the map
marker keeps its dark-fill-plus-white-outline look, while the details
pill uses a plain white fill (strokeColor: null) to match the existing
monochrome flag/ruler/clock icons instead of looking like a dropped-in
map pin.

app/lib/domain/mission/mission_stats.dart

@@ -136,3 +136,16 @@ LatLng? computeRubberBandTarget({
   final target = waypoints[activeWaypointIndex];
   return LatLng(target.lat, target.lon);
 }
+
+/// Entfernung von der aktuellen Position zum Homepoint in Metern (Doku:
+/// "Entfernung zum Home Punkt als letztes Element in der Details-Pille im
+/// Fly-Modus anzeigen") - null, solange eine der beiden Positionen nicht
+/// bekannt ist (kein GPS-Fix bzw. Homepoint noch nicht abgefragt, siehe
+/// home_point_provider.dart).
+double? computeDistanceToHomeM({
+  required LatLng? currentPosition,
+  required LatLng? homePoint,
+}) {
+  if (currentPosition == null || homePoint == null) return null;
+  return _distance(currentPosition, homePoint);
+}