Add center reticle control for dropping waypoints

- ReticleButton: fixed-at-screen-center "+" circle (HTML prototype:
  #centerCluster/#reticleBtn), tapping it drops a waypoint at the current
  map center.
- CurrentMissionNotifier (Riverpod): holds the waypoint list for the
  mission being edited. Reactive single-value state per doc 4.16, so
  Riverpod rather than a new Bloc/Cubit.
- MissionMap now renders each waypoint as a CircleLayer marker (white
  border, dark fill, matching the prototype's L.circleMarker styling).
- PlanScreen owns the MapController needed to read the camera center on
  tap, and wires the reticle to the provider.

Default altitude/speed/catch-radius (60m / 15 m/s / 60m) are hardcoded to
the T1 Ranger default profile values from the prototype for now - TODO
once drone profile settings (doc 3.5) exist.

Verified with flutter analyze, flutter test (incl. a new test covering the
add-waypoint path), and a real run on the Pixel_10a emulator: panned the
map and dropped two waypoints, both markers stayed correctly georeferenced.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

app/lib/ui/providers/current_mission_provider.dart

@@ -0,0 +1,21 @@
+import 'package:flutter_riverpod/flutter_riverpod.dart';
+
+import '../../domain/waypoint/flat_waypoint_list.dart';
+
+/// Haelt die Wegpunktliste der aktuell in Bearbeitung befindlichen Mission
+/// (Architektur-Doku 3.2/3.6). Reaktiver Einzelwert im Sinne von 4.16 -
+/// daher Riverpod statt Bloc; die App-Mode State Machine bleibt in
+/// AppModeCubit.
+class CurrentMissionNotifier extends Notifier<List<Waypoint>> {
+  @override
+  List<Waypoint> build() => [];
+
+  void addWaypoint(Waypoint waypoint) {
+    state = [...state, waypoint];
+  }
+}
+
+final currentMissionProvider =
+    NotifierProvider<CurrentMissionNotifier, List<Waypoint>>(
+  CurrentMissionNotifier.new,
+);

app/lib/ui/screens/plan/plan_screen.dart

@@ -1,14 +1,77 @@
 import 'package:flutter/material.dart';
+import 'package:flutter_map/flutter_map.dart';
+import 'package:flutter_riverpod/flutter_riverpod.dart';
+import 'package:latlong2/latlong.dart';
 
+import '../../../domain/waypoint/flat_waypoint_list.dart';
+import '../../providers/current_mission_provider.dart';
 import '../../widgets/mission_map.dart';
+import '../../widgets/reticle_button.dart';
 
 /// Missionsplanung: Karte + Wegpunktliste (Architektur-Doku 3.6/3.7).
 /// Kein eigenes Scaffold - die Kopfleiste (Plan/Fly-Umschalter) und der
 /// gemeinsame Scaffold-Rahmen liegen im AppShell darueber. Wegpunktliste,
 /// Draeder und Bottom-Bar folgen als naechster Schritt.
-class PlanScreen extends StatelessWidget {
+class PlanScreen extends ConsumerStatefulWidget {
   const PlanScreen({super.key});
 
   @override
-  Widget build(BuildContext context) => const MissionMap();
+  ConsumerState<PlanScreen> createState() => _PlanScreenState();
+}
+
+class _PlanScreenState extends ConsumerState<PlanScreen> {
+  final _mapController = MapController();
+
+  // TODO: aus dem aktiven Drohnen-Profil (Doku 3.5) uebernehmen, sobald die
+  // Profilverwaltung existiert. Werte entsprechen dem Default-Profil
+  // T1 Ranger im HTML-Demonstrator (curAlt=60, cruiseSpeed=15, catchRadius=60).
+  static const _defaultAltitudeM = 60.0;
+  static const _defaultSpeedMs = 15.0;
+  static const _defaultCatchRadiusM = 60.0;
+
+  @override
+  void dispose() {
+    _mapController.dispose();
+    super.dispose();
+  }
+
+  void _addWaypointAtMapCenter() {
+    final center = _mapController.camera.center;
+    ref.read(currentMissionProvider.notifier).addWaypoint(
+          Waypoint(
+            lat: center.latitude,
+            lon: center.longitude,
+            altitudeM: _defaultAltitudeM,
+            speedMs: _defaultSpeedMs,
+            catchRadiusM: _defaultCatchRadiusM,
+          ),
+        );
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    final waypoints = ref.watch(currentMissionProvider);
+
+    return Stack(
+      children: [
+        MissionMap(
+          mapController: _mapController,
+          waypointMarkers: [
+            for (final wp in waypoints)
+              CircleMarker(
+                point: LatLng(wp.lat, wp.lon),
+                radius: 6,
+                color: const Color(0xFF2B2B28),
+                borderStrokeWidth: 1.5,
+                borderColor: Colors.white,
+              ),
+          ],
+        ),
+        Align(
+          alignment: Alignment.center,
+          child: ReticleButton(onTap: _addWaypointAtMapCenter),
+        ),
+      ],
+    );
+  }
 }

app/lib/ui/widgets/mission_map.dart

@@ -6,10 +6,13 @@ import 'package:url_launcher/url_launcher.dart';
 /// Vollbild-OSM-Karte, entspricht dem Leaflet-`#map` im HTML-Demonstrator:
 /// randlos, ohne Zoom-Buttons (Pinch-to-Zoom/Pan reicht), Standard-OSM-Tiles.
 class MissionMap extends StatelessWidget {
-  const MissionMap({super.key, this.mapController});
+  const MissionMap({super.key, this.mapController, this.waypointMarkers = const []});
 
   final MapController? mapController;
 
+  /// Kreis-Marker fuer die Wegpunkte der aktuellen Mission (Doku 3.7).
+  final List<CircleMarker> waypointMarkers;
+
   // Default-Zentrum/Zoom uebernommen aus dem HTML-Demonstrator
   // (Drone Mission Control.html: L.map(...).setView([52.3676, 4.9041], 16)).
   static const _initialCenter = LatLng(52.3676, 4.9041);
@@ -31,6 +34,7 @@ class MissionMap extends StatelessWidget {
           userAgentPackageName: 'de.dmc.dmc_app',
           maxZoom: 19,
         ),
+        CircleLayer(circles: waypointMarkers),
         SimpleAttributionWidget(
           alignment: Alignment.bottomLeft,
           source: const Text('OpenStreetMap contributors'),

app/lib/ui/widgets/reticle_button.dart

@@ -0,0 +1,28 @@
+import 'package:flutter/material.dart';
+
+/// Zentrales Steuerelement zum Setzen von Wegpunkten (HTML-Demonstrator:
+/// #centerCluster/#reticleBtn), fixiert in der Bildschirmmitte. Die Karte
+/// wird darunter verschoben; ein Tap setzt einen Wegpunkt an der aktuellen
+/// Kartenmitte.
+class ReticleButton extends StatelessWidget {
+  const ReticleButton({super.key, required this.onTap});
+
+  final VoidCallback onTap;
+
+  @override
+  Widget build(BuildContext context) {
+    return GestureDetector(
+      onTap: onTap,
+      child: Container(
+        width: 55,
+        height: 55,
+        decoration: BoxDecoration(
+          shape: BoxShape.circle,
+          color: Colors.black.withValues(alpha: 0.68),
+          border: Border.all(color: Colors.white, width: 2),
+        ),
+        child: const Icon(Icons.add, color: Colors.white, size: 26),
+      ),
+    );
+  }
+}

app/test/widget_test.dart

@@ -6,6 +6,8 @@ import 'package:flutter_test/flutter_test.dart';
 
 import 'package:dmc_app/app_mode/app_mode_cubit.dart';
 import 'package:dmc_app/ui/app_shell.dart';
+import 'package:dmc_app/ui/providers/current_mission_provider.dart';
+import 'package:dmc_app/ui/widgets/reticle_button.dart';
 
 Widget _wrap(Widget child) => ProviderScope(
       child: BlocProvider(
@@ -43,4 +45,31 @@ void main() {
     // Weiterhin im Plan-Modus, da das Gate nicht erfuellt war.
     expect(find.byType(FlutterMap), findsOneWidget);
   });
+
+  testWidgets('Reticle-Button fuegt einen Wegpunkt am Kartenzentrum hinzu',
+      (tester) async {
+    final container = ProviderContainer();
+    addTearDown(container.dispose);
+
+    await tester.pumpWidget(
+      UncontrolledProviderScope(
+        container: container,
+        child: BlocProvider(
+          create: (_) => AppModeCubit(),
+          child: const MaterialApp(home: AppShell()),
+        ),
+      ),
+    );
+    await tester.pump();
+
+    expect(container.read(currentMissionProvider), isEmpty);
+
+    await tester.tap(find.byType(ReticleButton));
+    await tester.pump();
+
+    final waypoints = container.read(currentMissionProvider);
+    expect(waypoints, hasLength(1));
+    expect(waypoints.single.lat, closeTo(52.3676, 0.0001));
+    expect(waypoints.single.lon, closeTo(4.9041, 0.0001));
+  });
 }