Add Flutter project skeleton (Android-only MVP)

Five-layer architecture per DMC_Architektur_und_Design.md: FlightControllerLink
abstraction (Mock + MSP stub), protocol-neutral mission domain layer, App-Mode
state machine with Ready-to-Fly gate, Drift persistence schema, and placeholder
UI screens wired via go_router. App icon generated from design/Y2_pink_dark.svg.
Verified with flutter analyze and flutter test.

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

app/lib/ui/app_shell.dart

@@ -0,0 +1,19 @@
+import 'package:flutter/material.dart';
+import 'package:flutter_bloc/flutter_bloc.dart';
+
+import '../app_mode/app_mode_cubit.dart';
+import 'screens/fly/fly_screen.dart';
+import 'screens/plan/plan_screen.dart';
+
+/// Schaltet zwischen Plan- und Fly-Screen um, gesteuert von der
+/// App-Mode State Machine (Architektur-Doku 2.2). Connect/Upload sind
+/// Uebergangszustaende innerhalb des Plan-Screens (Handover-Prozess).
+class AppShell extends StatelessWidget {
+  const AppShell({super.key});
+
+  @override
+  Widget build(BuildContext context) {
+    final mode = context.select((AppModeCubit c) => c.state.mode);
+    return mode == AppMode.fly ? const FlyScreen() : const PlanScreen();
+  }
+}