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/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));
+  });
 }