Add location search, home/fit map buttons, and slim OSM attribution

- TopNavBar: Nominatim geocoding search field plus Home (center on first
  waypoint) and Fit (zoom to mission bounds) buttons (HTML prototype:
  #searchWrap/#homeBtn/#fitBtn). Both show a SnackBar instead of doing
  nothing when there are no waypoints yet.
- Renders as its own bar just below TopModeBar rather than fused into the
  same pill: TopModeBar lives in AppShell, a layer above PlanScreen, which
  owns the MapController these actions actually need. Fusing them would
  need lifting the MapController to shared state - reasonable follow-up
  if pixel-fidelity with the prototype's single bar matters later, but
  not necessary for the functionality itself.
- Replaced SimpleAttributionWidget with a compact custom attribution: OSM's
  tile usage policy requires visible attribution to stay, so instead of
  removing it (as first asked) I shrank it and dropped the "flutter_map |"
  prefix per the user's follow-up choice, so it reads cleanly against the
  now-opaque bottom bar instead of looking like a second banner.

Added 5 tests (nav bar renders, home/fit SnackBars with no waypoints,
empty search is a no-op). All 26 tests and flutter analyze pass. Verified
on the Pixel_10a emulator: searched "Berlin" and confirmed the map flew
there, dropped a waypoint, panned far away, and confirmed Home re-centers
and snaps onto it exactly.

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

app/test/widget_test.dart

@@ -11,6 +11,7 @@ import 'package:dmc_app/ui/providers/current_mission_provider.dart';
 import 'package:dmc_app/ui/widgets/full_value_chart.dart';
 import 'package:dmc_app/ui/widgets/halo_menu.dart';
 import 'package:dmc_app/ui/widgets/reticle_button.dart';
+import 'package:dmc_app/ui/widgets/top_nav_bar.dart';
 import 'package:dmc_app/ui/widgets/waypoint_chip.dart';
 import 'package:dmc_app/ui/widgets/waypoint_list_panel.dart';
 
@@ -309,4 +310,46 @@ void main() {
     final waypoints = container.read(currentMissionProvider);
     expect(waypoints.single.speedMs, greaterThan(15));
   });
+
+  testWidgets('TopNavBar zeigt Suchfeld und Home-/Fit-Buttons',
+      (tester) async {
+    await _pumpWithContainer(tester);
+
+    expect(find.byType(TopNavBar), findsOneWidget);
+    expect(find.text('Search location'), findsOneWidget);
+    expect(find.byIcon(Icons.home), findsOneWidget);
+    expect(find.byIcon(Icons.crop_free), findsOneWidget);
+  });
+
+  testWidgets('Home-Button ohne Wegpunkte zeigt Hinweis statt Absturz',
+      (tester) async {
+    await _pumpWithContainer(tester);
+
+    await tester.tap(find.byIcon(Icons.home));
+    await tester.pump();
+
+    expect(find.text('No waypoints yet'), findsOneWidget);
+  });
+
+  testWidgets('Fit-Button ohne Wegpunkte zeigt Hinweis statt Absturz',
+      (tester) async {
+    await _pumpWithContainer(tester);
+
+    await tester.tap(find.byIcon(Icons.crop_free));
+    await tester.pump();
+
+    expect(find.text('No waypoints to fit'), findsOneWidget);
+  });
+
+  testWidgets('Leere Suche loest keinen Netzwerkaufruf/Fehler aus',
+      (tester) async {
+    await _pumpWithContainer(tester);
+
+    await tester.enterText(find.byType(TextField), '');
+    await tester.testTextInput.receiveAction(TextInputAction.search);
+    await tester.pump();
+
+    expect(find.text('Search failed'), findsNothing);
+    expect(find.text('Location not found'), findsNothing);
+  });
 }