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/screens/plan/plan_screen.dart

@@ -0,0 +1,15 @@
+import 'package:flutter/material.dart';
+
+/// Missionsplanung: Karte + Wegpunktliste (Architektur-Doku 3.6/3.7).
+/// Kartenintegration (flutter_map) und Fillet-Geometrie folgen als naechster
+/// Schritt - dies ist bewusst nur der Screen-Platzhalter.
+class PlanScreen extends StatelessWidget {
+  const PlanScreen({super.key});
+
+  @override
+  Widget build(BuildContext context) {
+    return const Scaffold(
+      body: Center(child: Text('Plan')),
+    );
+  }
+}