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/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(
-        child: BlocProvider(
-          create: (_) => AppModeCubit(),
-          child: const MaterialApp(home: AppShell()),
-        ),
+Widget _wrap(Widget child) => ProviderScope(
+      child: BlocProvider(
+        create: (_) => AppModeCubit(),
+        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);
   });
 }