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