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/ui/widgets/mission_footer_bar.dart

@@ -24,6 +24,7 @@ class MissionFooterBar extends ConsumerWidget {
     required this.totalDistanceM,
     required this.totalDurationSec,
     this.flyMissionStatus,
+    this.distanceToHomeM,
     this.droneStatus,
     required this.warnings,
     this.showWarningsBanner = true,
@@ -48,6 +49,11 @@ class MissionFooterBar extends ConsumerWidget {
   /// Distanz/Zeit dorthin. Bleibt im Plan-Modus null.
   final FlyMissionStatus? flyMissionStatus;
 
+  /// Nur im Fly-Modus gesetzt (Doku: "zeige die Entfernung zum Home Punkt
+  /// als letztes element in der details pille im fly modus an") - siehe
+  /// BottomStatsBar.distanceToHomeM.
+  final double? distanceToHomeM;
+
   /// Nur im Fly-Modus gesetzt (Doku: "im fly modus zeigt die drone pille
   /// den zustand der drohne") - siehe BottomStatsBar.droneStatus. FlyScreen
   /// setzt hier immer einen Wert (auch ohne Telemetrie-Frame, siehe dortige
@@ -127,6 +133,7 @@ class MissionFooterBar extends ConsumerWidget {
                 totalDistanceM: totalDistanceM,
                 totalDurationSec: totalDurationSec,
                 flyMissionStatus: flyMissionStatus,
+                distanceToHomeM: distanceToHomeM,
                 droneStatus: droneStatus,
                 onTap: onDetailsTap,
                 warnings: warnings,