fix(#565): surface add-contact failures instead of a dead tap

Gemini pre-PR review finding. importDiscoveredContact no-ops when the radio
is gone, and sendFrame throws on a mid-write disconnect or an unwritable BLE
characteristic. Both reached the user as a tap that appeared to work.

Checks the connection first, catches the write failure, logs it, and shows a
persistent error snackbar in either case (SAFELANE section 6: no silent
failures).

lib/l10n/app_en.arb

@@ -912,6 +912,15 @@
       }
     }
   },
+  "channel_senderAddNotConnected": "Not connected to a radio, so this sender can't be added yet",
+  "channel_senderAddFailed": "Couldn't add this sender: {error}",
+  "@channel_senderAddFailed": {
+    "placeholders": {
+      "error": {
+        "type": "String"
+      }
+    }
+  },
   "channel_senderCandidate": "{keyPrefix} · {type}",
   "@channel_senderCandidate": {
     "placeholders": {

lib/l10n/app_localizations.dart

@@ -3154,6 +3154,18 @@ abstract class AppLocalizations {
   /// **'{count} nodes use this name'**
   String channel_senderMultipleMatches(int count);
 
+  /// No description provided for @channel_senderAddNotConnected.
+  ///
+  /// In en, this message translates to:
+  /// **'Not connected to a radio, so this sender can\'t be added yet'**
+  String get channel_senderAddNotConnected;
+
+  /// No description provided for @channel_senderAddFailed.
+  ///
+  /// In en, this message translates to:
+  /// **'Couldn\'t add this sender: {error}'**
+  String channel_senderAddFailed(String error);
+
   /// No description provided for @channel_senderCandidate.
   ///
   /// In en, this message translates to:

lib/l10n/app_localizations_bg.dart

@@ -1756,6 +1756,15 @@ class AppLocalizationsBg extends AppLocalizations {
     return '$count nodes use this name';
   }
 
+  @override
+  String get channel_senderAddNotConnected =>
+      'Not connected to a radio, so this sender can\'t be added yet';
+
+  @override
+  String channel_senderAddFailed(String error) {
+    return 'Couldn\'t add this sender: $error';
+  }
+
   @override
   String channel_senderCandidate(String keyPrefix, String type) {
     return '$keyPrefix · $type';

lib/l10n/app_localizations_de.dart

@@ -1754,6 +1754,15 @@ class AppLocalizationsDe extends AppLocalizations {
     return '$count nodes use this name';
   }
 
+  @override
+  String get channel_senderAddNotConnected =>
+      'Not connected to a radio, so this sender can\'t be added yet';
+
+  @override
+  String channel_senderAddFailed(String error) {
+    return 'Couldn\'t add this sender: $error';
+  }
+
   @override
   String channel_senderCandidate(String keyPrefix, String type) {
     return '$keyPrefix · $type';

lib/l10n/app_localizations_en.dart

@@ -1725,6 +1725,15 @@ class AppLocalizationsEn extends AppLocalizations {
     return '$count nodes use this name';
   }
 
+  @override
+  String get channel_senderAddNotConnected =>
+      'Not connected to a radio, so this sender can\'t be added yet';
+
+  @override
+  String channel_senderAddFailed(String error) {
+    return 'Couldn\'t add this sender: $error';
+  }
+
   @override
   String channel_senderCandidate(String keyPrefix, String type) {
     return '$keyPrefix · $type';

lib/l10n/app_localizations_es.dart

@@ -1751,6 +1751,15 @@ class AppLocalizationsEs extends AppLocalizations {
     return '$count nodes use this name';
   }
 
+  @override
+  String get channel_senderAddNotConnected =>
+      'Not connected to a radio, so this sender can\'t be added yet';
+
+  @override
+  String channel_senderAddFailed(String error) {
+    return 'Couldn\'t add this sender: $error';
+  }
+
   @override
   String channel_senderCandidate(String keyPrefix, String type) {
     return '$keyPrefix · $type';

lib/l10n/app_localizations_fr.dart

@@ -1758,6 +1758,15 @@ class AppLocalizationsFr extends AppLocalizations {
     return '$count nodes use this name';
   }
 
+  @override
+  String get channel_senderAddNotConnected =>
+      'Not connected to a radio, so this sender can\'t be added yet';
+
+  @override
+  String channel_senderAddFailed(String error) {
+    return 'Couldn\'t add this sender: $error';
+  }
+
   @override
   String channel_senderCandidate(String keyPrefix, String type) {
     return '$keyPrefix · $type';

lib/l10n/app_localizations_hu.dart

@@ -1763,6 +1763,15 @@ class AppLocalizationsHu extends AppLocalizations {
     return '$count nodes use this name';
   }
 
+  @override
+  String get channel_senderAddNotConnected =>
+      'Not connected to a radio, so this sender can\'t be added yet';
+
+  @override
+  String channel_senderAddFailed(String error) {
+    return 'Couldn\'t add this sender: $error';
+  }
+
   @override
   String channel_senderCandidate(String keyPrefix, String type) {
     return '$keyPrefix · $type';

lib/l10n/app_localizations_it.dart

@@ -1753,6 +1753,15 @@ class AppLocalizationsIt extends AppLocalizations {
     return '$count nodes use this name';
   }
 
+  @override
+  String get channel_senderAddNotConnected =>
+      'Not connected to a radio, so this sender can\'t be added yet';
+
+  @override
+  String channel_senderAddFailed(String error) {
+    return 'Couldn\'t add this sender: $error';
+  }
+
   @override
   String channel_senderCandidate(String keyPrefix, String type) {
     return '$keyPrefix · $type';

lib/l10n/app_localizations_ja.dart

@@ -1688,6 +1688,15 @@ class AppLocalizationsJa extends AppLocalizations {
     return '$count nodes use this name';
   }
 
+  @override
+  String get channel_senderAddNotConnected =>
+      'Not connected to a radio, so this sender can\'t be added yet';
+
+  @override
+  String channel_senderAddFailed(String error) {
+    return 'Couldn\'t add this sender: $error';
+  }
+
   @override
   String channel_senderCandidate(String keyPrefix, String type) {
     return '$keyPrefix · $type';

lib/l10n/app_localizations_ko.dart

@@ -1684,6 +1684,15 @@ class AppLocalizationsKo extends AppLocalizations {
     return '$count nodes use this name';
   }
 
+  @override
+  String get channel_senderAddNotConnected =>
+      'Not connected to a radio, so this sender can\'t be added yet';
+
+  @override
+  String channel_senderAddFailed(String error) {
+    return 'Couldn\'t add this sender: $error';
+  }
+
   @override
   String channel_senderCandidate(String keyPrefix, String type) {
     return '$keyPrefix · $type';

lib/l10n/app_localizations_nl.dart

@@ -1741,6 +1741,15 @@ class AppLocalizationsNl extends AppLocalizations {
     return '$count nodes use this name';
   }
 
+  @override
+  String get channel_senderAddNotConnected =>
+      'Not connected to a radio, so this sender can\'t be added yet';
+
+  @override
+  String channel_senderAddFailed(String error) {
+    return 'Couldn\'t add this sender: $error';
+  }
+
   @override
   String channel_senderCandidate(String keyPrefix, String type) {
     return '$keyPrefix · $type';

lib/l10n/app_localizations_pl.dart

@@ -1766,6 +1766,15 @@ class AppLocalizationsPl extends AppLocalizations {
     return '$count nodes use this name';
   }
 
+  @override
+  String get channel_senderAddNotConnected =>
+      'Not connected to a radio, so this sender can\'t be added yet';
+
+  @override
+  String channel_senderAddFailed(String error) {
+    return 'Couldn\'t add this sender: $error';
+  }
+
   @override
   String channel_senderCandidate(String keyPrefix, String type) {
     return '$keyPrefix · $type';

lib/l10n/app_localizations_pt.dart

@@ -1750,6 +1750,15 @@ class AppLocalizationsPt extends AppLocalizations {
     return '$count nodes use this name';
   }
 
+  @override
+  String get channel_senderAddNotConnected =>
+      'Not connected to a radio, so this sender can\'t be added yet';
+
+  @override
+  String channel_senderAddFailed(String error) {
+    return 'Couldn\'t add this sender: $error';
+  }
+
   @override
   String channel_senderCandidate(String keyPrefix, String type) {
     return '$keyPrefix · $type';

lib/l10n/app_localizations_ru.dart

@@ -1754,6 +1754,15 @@ class AppLocalizationsRu extends AppLocalizations {
     return '$count nodes use this name';
   }
 
+  @override
+  String get channel_senderAddNotConnected =>
+      'Not connected to a radio, so this sender can\'t be added yet';
+
+  @override
+  String channel_senderAddFailed(String error) {
+    return 'Couldn\'t add this sender: $error';
+  }
+
   @override
   String channel_senderCandidate(String keyPrefix, String type) {
     return '$keyPrefix · $type';

lib/l10n/app_localizations_sk.dart

@@ -1743,6 +1743,15 @@ class AppLocalizationsSk extends AppLocalizations {
     return '$count nodes use this name';
   }
 
+  @override
+  String get channel_senderAddNotConnected =>
+      'Not connected to a radio, so this sender can\'t be added yet';
+
+  @override
+  String channel_senderAddFailed(String error) {
+    return 'Couldn\'t add this sender: $error';
+  }
+
   @override
   String channel_senderCandidate(String keyPrefix, String type) {
     return '$keyPrefix · $type';

lib/l10n/app_localizations_sl.dart

@@ -1739,6 +1739,15 @@ class AppLocalizationsSl extends AppLocalizations {
     return '$count nodes use this name';
   }
 
+  @override
+  String get channel_senderAddNotConnected =>
+      'Not connected to a radio, so this sender can\'t be added yet';
+
+  @override
+  String channel_senderAddFailed(String error) {
+    return 'Couldn\'t add this sender: $error';
+  }
+
   @override
   String channel_senderCandidate(String keyPrefix, String type) {
     return '$keyPrefix · $type';

lib/l10n/app_localizations_sv.dart

@@ -1734,6 +1734,15 @@ class AppLocalizationsSv extends AppLocalizations {
     return '$count nodes use this name';
   }
 
+  @override
+  String get channel_senderAddNotConnected =>
+      'Not connected to a radio, so this sender can\'t be added yet';
+
+  @override
+  String channel_senderAddFailed(String error) {
+    return 'Couldn\'t add this sender: $error';
+  }
+
   @override
   String channel_senderCandidate(String keyPrefix, String type) {
     return '$keyPrefix · $type';

lib/l10n/app_localizations_uk.dart

@@ -1749,6 +1749,15 @@ class AppLocalizationsUk extends AppLocalizations {
     return '$count nodes use this name';
   }
 
+  @override
+  String get channel_senderAddNotConnected =>
+      'Not connected to a radio, so this sender can\'t be added yet';
+
+  @override
+  String channel_senderAddFailed(String error) {
+    return 'Couldn\'t add this sender: $error';
+  }
+
   @override
   String channel_senderCandidate(String keyPrefix, String type) {
     return '$keyPrefix · $type';

lib/l10n/app_localizations_zh.dart

@@ -1663,6 +1663,15 @@ class AppLocalizationsZh extends AppLocalizations {
     return '$count nodes use this name';
   }
 
+  @override
+  String get channel_senderAddNotConnected =>
+      'Not connected to a radio, so this sender can\'t be added yet';
+
+  @override
+  String channel_senderAddFailed(String error) {
+    return 'Couldn\'t add this sender: $error';
+  }
+
   @override
   String channel_senderCandidate(String keyPrefix, String type) {
     return '$keyPrefix · $type';

lib/screens/channel_chat_screen.dart

@@ -11,6 +11,7 @@ import 'package:provider/provider.dart';
 import '../connector/meshcore_connector.dart';
 import '../models/community.dart';
 import '../storage/community_store.dart';
+import '../utils/app_logger.dart';
 import '../utils/platform_info.dart';
 import '../helpers/chat_scroll_controller.dart';
 import '../connector/meshcore_protocol.dart';
@@ -797,12 +798,35 @@ class _ChannelChatScreenState extends State<ChannelChatScreen> {
 
   Future<void> _addSenderContact(Contact candidate) async {
     final connector = context.read<MeshCoreConnector>();
-    final messenger = ScaffoldMessenger.of(context);
     final l10n = context.l10n;
-    await connector.importDiscoveredContact(candidate);
+    // importDiscoveredContact silently no-ops without a radio, and sendFrame
+    // throws on a mid-write disconnect. Neither may reach the user as a tap
+    // that just did nothing (#565).
+    if (!connector.isConnected) {
+      _showSenderError(l10n.channel_senderAddNotConnected);
+      return;
+    }
+    try {
+      await connector.importDiscoveredContact(candidate);
+    } catch (e) {
+      appLogger.error('Adding channel sender ${candidate.name} failed: $e');
+      if (!mounted) return;
+      _showSenderError(l10n.channel_senderAddFailed(e.toString()));
+      return;
+    }
     if (!mounted) return;
-    messenger.showSnackBar(
-      SnackBar(content: Text(l10n.discoveredContacts_contactAdded)),
+    showDismissibleSnackBar(
+      context,
+      content: Text(l10n.discoveredContacts_contactAdded),
+    );
+  }
+
+  void _showSenderError(String message) {
+    showDismissibleSnackBar(
+      context,
+      content: Text(message),
+      backgroundColor: Theme.of(context).colorScheme.error,
+      persist: true,
     );
   }