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/test/domain/mission_stats_test.dart

@@ -187,4 +187,29 @@ void main() {
       );
     });
   });
+
+  group('computeDistanceToHomeM', () {
+    test('berechnet die Distanz aus Grosskreis', () {
+      final distance = computeDistanceToHomeM(
+        currentPosition: const LatLng(0, 0),
+        homePoint: const LatLng(0, 0.01),
+      );
+      // 0.01 Grad Laengendifferenz am Aequator entspricht rund 1112 m.
+      expect(distance, closeTo(1112.6, 1));
+    });
+
+    test('null ohne aktuelle Position (kein GPS-Fix)', () {
+      expect(
+        computeDistanceToHomeM(currentPosition: null, homePoint: const LatLng(0, 0)),
+        isNull,
+      );
+    });
+
+    test('null ohne bekannten Homepoint', () {
+      expect(
+        computeDistanceToHomeM(currentPosition: const LatLng(0, 0), homePoint: null),
+        isNull,
+      );
+    });
+  });
 }