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/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),
+        ),
+      ],
+    );
+  }
 }