Lock app to landscape fullscreen and add first OSM map

- Lock orientation to landscape (sensorLandscape in the manifest,
  SystemChrome.setPreferredOrientations in main.dart) and enable immersive
  fullscreen, matching the prototype's full-bleed map UI (doc 7.7).
- Add MissionMap widget: flutter_map + standard OSM raster tiles, same
  default center/zoom as the HTML prototype (Amsterdam, zoom 16), OSM
  attribution bottom-left. PlanScreen now renders it full-screen instead of
  the placeholder text.
- Add the release-manifest INTERNET permission (previously only present in
  the debug manifest, which would have silently broken tile loading and any
  future network calls in release builds).
- Update the smoke test to check for the FlutterMap widget instead of the
  removed placeholder text.

Verified with flutter analyze, flutter test, and a real run on the
Pixel_10a emulator (fullscreen landscape confirmed, tiles loading).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

app/lib/main.dart

@@ -1,4 +1,5 @@
 import 'package:flutter/material.dart';
+import 'package:flutter/services.dart';
 import 'package:flutter_bloc/flutter_bloc.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
 import 'package:go_router/go_router.dart';
@@ -9,7 +10,17 @@ import 'ui/dmc_colors.dart';
 import 'ui/screens/library/library_screen.dart';
 import 'ui/screens/settings/settings_screen.dart';
 
-void main() {
+Future<void> main() async {
+  WidgetsFlutterBinding.ensureInitialized();
+
+  // App laeuft ausschliesslich im Landscape-Vollbild (Prototyp-Referenz:
+  // Kartenansicht nutzt den gesamten Bildschirm ohne System-Chrome).
+  await SystemChrome.setPreferredOrientations([
+    DeviceOrientation.landscapeLeft,
+    DeviceOrientation.landscapeRight,
+  ]);
+  await SystemChrome.setEnabledSystemUIMode(SystemUiMode.immersiveSticky);
+
   runApp(const DmcApp());
 }