fix(extensions): default verification browser to in-app first

Prefer the in-app browser for signed-session verification challenges,
normalize invalid saved modes to the new default, and keep the help
dialog modal until the user explicitly dismisses it.

lib/models/settings.dart

@@ -129,7 +129,7 @@ class AppSettings {
     this.singleFilenameFormat = '{title} - {artist}',
     this.albumFolderStructure = 'artist_album',
     this.showExtensionStore = true,
-    this.extensionVerificationBrowserMode = 'external_first',
+    this.extensionVerificationBrowserMode = 'in_app_first',
     this.locale = 'system',
     this.lyricsMode = 'embed',
     this.tidalHighFormat = 'mp3_320',

lib/models/settings.g.dart

@@ -49,7 +49,7 @@ AppSettings _$AppSettingsFromJson(Map<String, dynamic> json) => AppSettings(
       json['albumFolderStructure'] as String? ?? 'artist_album',
   showExtensionStore: json['showExtensionStore'] as bool? ?? true,
   extensionVerificationBrowserMode:
-      json['extensionVerificationBrowserMode'] as String? ?? 'external_first',
+      json['extensionVerificationBrowserMode'] as String? ?? 'in_app_first',
   locale: json['locale'] as String? ?? 'system',
   lyricsMode: json['lyricsMode'] as String? ?? 'embed',
   tidalHighFormat: json['tidalHighFormat'] as String? ?? 'mp3_320',

lib/providers/settings_provider.dart

@@ -283,7 +283,7 @@ class SettingsNotifier extends Notifier<AppSettings> {
     if (_extensionVerificationBrowserModeValues.contains(normalized)) {
       return normalized;
     }
-    return 'external_first';
+    return 'in_app_first';
   }
 
   String? _sanitizeRetiredBuiltInProviderId(String? providerId) {

lib/screens/settings/app_settings_page.dart

@@ -394,7 +394,9 @@ class _VerificationBrowserModeSelector extends StatelessWidget {
     final colorScheme = Theme.of(context).colorScheme;
     final normalizedMode = currentMode == 'in_app_first'
         ? 'in_app_first'
-        : 'external_first';
+        : currentMode == 'external_first'
+        ? 'external_first'
+        : 'in_app_first';
 
     return Column(
       mainAxisSize: MainAxisSize.min,

lib/utils/extension_auth_launcher.dart

@@ -35,7 +35,7 @@ bool _containsHttpStatusCode(String message, String code) {
 
 Future<bool> openPendingExtensionVerification(
   String extensionId, {
-  String browserMode = 'external_first',
+  String browserMode = 'in_app_first',
   void Function(Uri authUri)? onAuthUri,
 }) async {
   final normalizedExtensionId = extensionId.trim();
@@ -79,7 +79,7 @@ Future<bool> openPendingExtensionVerification(
 Timer? scheduleExtensionVerificationHelpDialog(
   String extensionId,
   Uri? authUri, {
-  String browserMode = 'external_first',
+  String browserMode = 'in_app_first',
   Duration delay = const Duration(seconds: 20),
 }) {
   final normalizedExtensionId = extensionId.trim();
@@ -99,7 +99,7 @@ Timer? scheduleExtensionVerificationHelpDialog(
 Future<bool> showExtensionVerificationHelpDialog(
   String extensionId,
   Uri authUri, {
-  String browserMode = 'external_first',
+  String browserMode = 'in_app_first',
   bool immediateFailure = false,
 }) async {
   final context = AppNavigationService.rootNavigatorKey.currentContext;
@@ -119,6 +119,7 @@ Future<bool> showExtensionVerificationHelpDialog(
   await showDialog<void>(
     context: context,
     useRootNavigator: true,
+    barrierDismissible: false,
     builder: (dialogContext) {
       final dialogL10n = dialogContext.l10n;
       return AlertDialog(