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,
+);