Add top bar for switching between Plan and Fly mode

- TopModeBar widget: persistent pill-shaped header (HTML prototype:
  #topBarWrap/#editModeBtn/#flyModeBtn) tinted in the active mode's color,
  with the inactive mode offered as a solid button in the corner to switch
  into it.
- AppShell now provides a single shared Scaffold + Stack, overlaying
  TopModeBar on top of whichever screen (Plan/Fly) is active, instead of
  each screen owning its own Scaffold.
- Wired the Fly button to AppModeCubit.toFly(), which enforces the
  Ready-to-Fly gate (doc 4.5). Since upload/verify isn't wired up yet, the
  gate is never satisfied yet, so tapping Fly now shows a SnackBar instead
  of throwing an uncaught StateError.
- Extended the widget test to cover both the bar's presence and the
  gate-rejection path (was the source of a real bug: an initial
  negative-margin Container hack for the "bleed into the corner" look
  violated Container's margin.isNonNegative assertion and crashed the
  whole tree - fixed with padding instead).

Verified with flutter analyze, flutter test, and a real run on the
Pixel_10a emulator (visually matches the prototype screenshots in design/,
Fly-tap correctly shows the gate message without crashing or switching mode).

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

app/lib/ui/app_shell.dart

@@ -4,9 +4,11 @@ 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';
+import 'widgets/top_mode_bar.dart';
 
-/// Schaltet zwischen Plan- und Fly-Screen um, gesteuert von der
-/// App-Mode State Machine (Architektur-Doku 2.2). Connect/Upload sind
+/// Haelt die persistente Kopfleiste (Plan/Fly-Umschalter) und schaltet
+/// darunter 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});
@@ -14,6 +16,15 @@ class AppShell extends StatelessWidget {
   @override
   Widget build(BuildContext context) {
     final mode = context.select((AppModeCubit c) => c.state.mode);
-    return mode == AppMode.fly ? const FlyScreen() : const PlanScreen();
+    final screen = mode == AppMode.fly ? const FlyScreen() : const PlanScreen();
+
+    return Scaffold(
+      body: Stack(
+        children: [
+          Positioned.fill(child: screen),
+          const Align(alignment: Alignment.topCenter, child: TopModeBar()),
+        ],
+      ),
+    );
   }
 }

app/lib/ui/screens/fly/fly_screen.dart

@@ -2,14 +2,18 @@ import 'package:flutter/material.dart';
 
 /// Fly-Modus: Telemetrie, Draeder, Point-and-Fly-Fadenkreuz
 /// (Architektur-Doku 3.10/3.11). Platzhalter bis Telemetry-Service und
-/// FlightControllerLink verdrahtet sind.
+/// FlightControllerLink verdrahtet sind. Kein eigenes Scaffold, siehe
+/// PlanScreen/AppShell.
 class FlyScreen extends StatelessWidget {
   const FlyScreen({super.key});
 
   @override
   Widget build(BuildContext context) {
-    return const Scaffold(
-      body: Center(child: Text('Fly')),
+    return const ColoredBox(
+      color: Color(0xFF0F0F0F),
+      child: Center(
+        child: Text('Fly', style: TextStyle(color: Colors.white)),
+      ),
     );
   }
 }

app/lib/ui/screens/plan/plan_screen.dart

@@ -3,16 +3,12 @@ import 'package:flutter/material.dart';
 import '../../widgets/mission_map.dart';
 
 /// Missionsplanung: Karte + Wegpunktliste (Architektur-Doku 3.6/3.7).
-/// Top-/Bottom-Bar-Overlays (Modus-Umschalter, Draeder, Wegpunktliste) folgen
-/// als naechster Schritt - dies ist bewusst erst die Vollbild-Karte.
+/// Kein eigenes Scaffold - die Kopfleiste (Plan/Fly-Umschalter) und der
+/// gemeinsame Scaffold-Rahmen liegen im AppShell darueber. Wegpunktliste,
+/// Draeder und Bottom-Bar folgen als naechster Schritt.
 class PlanScreen extends StatelessWidget {
   const PlanScreen({super.key});
 
   @override
-  Widget build(BuildContext context) {
-    return const Scaffold(
-      extendBodyBehindAppBar: true,
-      body: MissionMap(),
-    );
-  }
+  Widget build(BuildContext context) => const MissionMap();
 }

app/lib/ui/widgets/top_mode_bar.dart

@@ -0,0 +1,116 @@
+import 'package:flutter/material.dart';
+import 'package:flutter_bloc/flutter_bloc.dart';
+
+import '../../app_mode/app_mode_cubit.dart';
+import '../dmc_colors.dart';
+
+/// Persistente Kopfleiste zum Umschalten zwischen Plan- und Fly-Modus
+/// (HTML-Demonstrator: #topBarWrap/#editModeBtn/#flyModeBtn). Die Leiste
+/// faerbt sich in der Farbe des aktiven Modus; der jeweils andere Modus wird
+/// als solider Button am Rand angeboten, um in ihn zu wechseln.
+class TopModeBar extends StatelessWidget {
+  const TopModeBar({super.key});
+
+  @override
+  Widget build(BuildContext context) {
+    final isFly = context.select((AppModeCubit c) => c.state.mode == AppMode.fly);
+    final activeTint = isFly ? DmcColors.flyModeRed : DmcColors.editModeGreen;
+
+    return SafeArea(
+      bottom: false,
+      child: FractionallySizedBox(
+        widthFactor: 0.64,
+        child: Container(
+          padding: const EdgeInsets.symmetric(horizontal: 10, vertical: 5),
+          decoration: BoxDecoration(
+            color: activeTint.withValues(alpha: 0.55),
+            borderRadius: const BorderRadius.only(
+              bottomLeft: Radius.circular(26),
+              bottomRight: Radius.circular(26),
+            ),
+          ),
+          child: Row(
+            children: [
+              _ModeButton(
+                label: 'Plan',
+                isActive: !isFly,
+                inactiveColor: DmcColors.editModeGreen,
+                cornerRadius: const BorderRadius.only(
+                  bottomLeft: Radius.circular(26),
+                ),
+                onTap: () => context.read<AppModeCubit>().toPlan(),
+              ),
+              const Spacer(),
+              _ModeButton(
+                label: 'Fly',
+                isActive: isFly,
+                inactiveColor: DmcColors.flyModeRed,
+                cornerRadius: const BorderRadius.only(
+                  bottomRight: Radius.circular(26),
+                ),
+                onTap: () => _tryEnterFly(context),
+              ),
+            ],
+          ),
+        ),
+      ),
+    );
+  }
+
+  void _tryEnterFly(BuildContext context) {
+    try {
+      context.read<AppModeCubit>().toFly();
+    } on StateError {
+      // Ready-to-Fly-Gate (Doku 2.2/4.5) nicht erfuellt - Upload/Verify-Flow
+      // ist noch nicht verdrahtet, daher aktuell immer dieser Fall.
+      ScaffoldMessenger.of(context).showSnackBar(
+        const SnackBar(
+          content: Text(
+            'Mission muss zuerst hochgeladen und verifiziert werden.',
+          ),
+        ),
+      );
+    }
+  }
+}
+
+class _ModeButton extends StatelessWidget {
+  const _ModeButton({
+    required this.label,
+    required this.isActive,
+    required this.inactiveColor,
+    required this.cornerRadius,
+    required this.onTap,
+  });
+
+  final String label;
+  final bool isActive;
+  final Color inactiveColor;
+  final BorderRadius cornerRadius;
+  final VoidCallback onTap;
+
+  @override
+  Widget build(BuildContext context) {
+    return GestureDetector(
+      onTap: isActive ? null : onTap,
+      child: Container(
+        padding: EdgeInsets.symmetric(
+          horizontal: 24,
+          vertical: isActive ? 6 : 11,
+        ),
+        decoration: BoxDecoration(
+          color: isActive ? null : inactiveColor.withValues(alpha: 0.85),
+          borderRadius: isActive ? null : cornerRadius,
+        ),
+        child: Text(
+          label,
+          style: const TextStyle(
+            fontSize: 20,
+            fontWeight: FontWeight.w800,
+            color: Color(0xFF141414),
+          ),
+        ),
+      ),
+    );
+  }
+}

app/test/widget_test.dart

@@ -7,20 +7,40 @@ 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';
 
-void main() {
-  testWidgets('AppShell zeigt die Karte im Plan-Startzustand', (tester) async {
-    await tester.pumpWidget(
-      ProviderScope(
+Widget _wrap(Widget child) => ProviderScope(
       child: BlocProvider(
         create: (_) => AppModeCubit(),
-          child: const MaterialApp(home: AppShell()),
-        ),
+        child: MaterialApp(home: child),
       ),
     );
+
+void main() {
+  testWidgets('AppShell zeigt die Karte und den Plan/Fly-Umschalter im Startzustand',
+      (tester) async {
+    await tester.pumpWidget(_wrap(const AppShell()));
     // Nur ein Frame pumpen statt pumpAndSettle: TileLayer laedt echte
     // Netzwerk-Kacheln, die in der Testumgebung nie ankommen.
     await tester.pump();
 
     expect(find.byType(FlutterMap), findsOneWidget);
+    expect(find.text('Plan'), findsOneWidget);
+    expect(find.text('Fly'), findsOneWidget);
+  });
+
+  testWidgets(
+      'Fly-Button ohne erfuellten Ready-to-Fly-Gate zeigt Hinweis statt Absturz',
+      (tester) async {
+    await tester.pumpWidget(_wrap(const AppShell()));
+    await tester.pump();
+
+    await tester.tap(find.text('Fly'));
+    await tester.pump();
+
+    expect(
+      find.text('Mission muss zuerst hochgeladen und verifiziert werden.'),
+      findsOneWidget,
+    );
+    // Weiterhin im Plan-Modus, da das Gate nicht erfuellt war.
+    expect(find.byType(FlutterMap), findsOneWidget);
   });
 }