fix: do not let a cold-start deep link become the router's location

lib/core/app_routes.dart

@@ -37,17 +37,24 @@ import 'package:mostro_mobile/features/walkthrough/providers/first_run_provider.
 import 'package:mostro_mobile/shared/widgets/navigation_listener_widget.dart';
 import 'package:mostro_mobile/shared/widgets/notification_listener_widget.dart';
 import 'package:mostro_mobile/generated/l10n.dart';
+import 'package:mostro_mobile/core/deep_link_interceptor.dart';
 import 'package:mostro_mobile/services/logger_service.dart';
 
 GoRouter createRouter(WidgetRef ref) {
+  // A cold-start deep link arrives as the platform default route, which
+  // go_router prefers over initialLocation; matching it asserts. Kept
+  // conditional so web still opens at the requested URL.
+  final platformDefaultLocation =
+      WidgetsBinding.instance.platformDispatcher.defaultRouteName;
+
   return GoRouter(
     navigatorKey: MostroApp.navigatorKey,
     initialLocation: '/',
+    overridePlatformDefaultLocation:
+        DeepLinkInterceptor.isCustomSchemeLocation(platformDefaultLocation),
     redirect: (context, state) {
       // Redirect custom schemes to home to prevent assertion failures
-      if (state.uri.scheme == 'mostro' ||
-          (!state.uri.scheme.startsWith('http') &&
-              state.uri.scheme.isNotEmpty)) {
+      if (DeepLinkInterceptor.isCustomSchemeUri(state.uri)) {
         return '/';
       }
       final firstRunState = ref.read(firstRunProvider);

lib/core/deep_link_interceptor.dart

@@ -62,9 +62,16 @@ class DeepLinkInterceptor extends WidgetsBindingObserver {
   }
 
   /// Check if the URI uses a custom scheme
-  bool _isCustomScheme(Uri uri) {
-    return uri.scheme == 'mostro' ||
-           (!uri.scheme.startsWith('http') && uri.scheme.isNotEmpty);
+  bool _isCustomScheme(Uri uri) => isCustomSchemeUri(uri);
+
+  /// Whether the URI uses a scheme the app resolves itself, such as `mostro:`
+  static bool isCustomSchemeUri(Uri uri) =>
+      uri.scheme.isNotEmpty && !uri.scheme.startsWith('http');
+
+  /// [isCustomSchemeUri] for an unparsed location; unparseable means no
+  static bool isCustomSchemeLocation(String location) {
+    final uri = Uri.tryParse(location);
+    return uri != null && isCustomSchemeUri(uri);
   }
 
   /// Dispose the interceptor

test/core/app_routes_test.dart

@@ -0,0 +1,85 @@
+import 'package:flutter/material.dart';
+import 'package:flutter_riverpod/flutter_riverpod.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:go_router/go_router.dart';
+import 'package:mostro_mobile/core/app_routes.dart';
+import 'package:mostro_mobile/shared/providers/storage_providers.dart';
+import 'package:shared_preferences/shared_preferences.dart';
+import 'package:shared_preferences_platform_interface/in_memory_shared_preferences_async.dart';
+import 'package:shared_preferences_platform_interface/shared_preferences_async_platform_interface.dart';
+
+const _mostroLink =
+    'mostro:8927bb1d-da68-491e-b0e2-db0ed548d52c?relays=wss://relay.mostro.network';
+
+/// Builds the app's real router inside a scope that can resolve it, and hands
+/// it back without mounting any screen.
+Future<GoRouter> buildRouter(WidgetTester tester) async {
+  late GoRouter router;
+  await tester.pumpWidget(
+    ProviderScope(
+      overrides: [
+        sharedPreferencesProvider.overrideWithValue(SharedPreferencesAsync()),
+      ],
+      child: Consumer(
+        builder: (context, ref, _) {
+          router = createRouter(ref);
+          return const SizedBox.shrink();
+        },
+      ),
+    ),
+  );
+  return router;
+}
+
+void main() {
+  setUp(() {
+    SharedPreferencesAsyncPlatform.instance =
+        InMemorySharedPreferencesAsync.empty();
+  });
+
+  group('createRouter initial location', () {
+    // Regression test for #670: go_router preferred the cold-start deep link
+    // over initialLocation and asserted while matching it.
+    testWidgets('ignores a custom scheme handed over by the platform',
+        (tester) async {
+      tester.binding.platformDispatcher.defaultRouteNameTestValue = _mostroLink;
+      addTearDown(
+          tester.binding.platformDispatcher.clearDefaultRouteNameTestValue);
+
+      final router = await buildRouter(tester);
+
+      expect(
+        router.routeInformationProvider.value.uri.toString(),
+        '/',
+      );
+      expect(tester.takeException(), isNull);
+    });
+
+    testWidgets('starts at the root on an ordinary launch', (tester) async {
+      tester.binding.platformDispatcher.defaultRouteNameTestValue = '/';
+      addTearDown(
+          tester.binding.platformDispatcher.clearDefaultRouteNameTestValue);
+
+      final router = await buildRouter(tester);
+
+      expect(router.routeInformationProvider.value.uri.toString(), '/');
+      expect(tester.takeException(), isNull);
+    });
+
+    // On web the platform default is a real location and must still win.
+    testWidgets('honours a real location handed over by the platform',
+        (tester) async {
+      tester.binding.platformDispatcher.defaultRouteNameTestValue = '/settings';
+      addTearDown(
+          tester.binding.platformDispatcher.clearDefaultRouteNameTestValue);
+
+      final router = await buildRouter(tester);
+
+      expect(
+        router.routeInformationProvider.value.uri.toString(),
+        '/settings',
+      );
+      expect(tester.takeException(), isNull);
+    });
+  });
+}

test/core/deep_link_interceptor_test.dart

@@ -0,0 +1,53 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:mostro_mobile/core/deep_link_interceptor.dart';
+
+void main() {
+  group('DeepLinkInterceptor.isCustomSchemeLocation', () {
+    test('claims mostro links', () {
+      expect(
+        DeepLinkInterceptor.isCustomSchemeLocation(
+          'mostro:8927bb1d-da68-491e-b0e2-db0ed548d52c'
+          '?relays=wss://relay.mostro.network',
+        ),
+        isTrue,
+      );
+      expect(DeepLinkInterceptor.isCustomSchemeLocation('mostro:'), isTrue);
+    });
+
+    test('claims other non-web schemes', () {
+      expect(
+        DeepLinkInterceptor.isCustomSchemeLocation('lightning:lnbc1...'),
+        isTrue,
+      );
+    });
+
+    test('leaves app locations alone', () {
+      expect(DeepLinkInterceptor.isCustomSchemeLocation('/'), isFalse);
+      expect(
+        DeepLinkInterceptor.isCustomSchemeLocation('/take_sell/order-1'),
+        isFalse,
+      );
+      expect(
+        DeepLinkInterceptor.isCustomSchemeLocation('/settings?tab=relays'),
+        isFalse,
+      );
+      expect(DeepLinkInterceptor.isCustomSchemeLocation(''), isFalse);
+    });
+
+    test('leaves web locations alone', () {
+      expect(
+        DeepLinkInterceptor.isCustomSchemeLocation('https://mostro.network/x'),
+        isFalse,
+      );
+      expect(
+        DeepLinkInterceptor.isCustomSchemeLocation('http://localhost:8080/'),
+        isFalse,
+      );
+    });
+
+    test('treats an unparseable location as an ordinary one', () {
+      // Nothing we could hand to the deep link handler either.
+      expect(DeepLinkInterceptor.isCustomSchemeLocation('::::'), isFalse);
+    });
+  });
+}