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/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'),