feat: add Codec2 voice messages over LoRa mesh (iOS/macOS only)

Push-to-talk voice messaging using the Codec2 ultra-low-bitrate speech
codec, transmitted as V: prefixed packets over the existing MeshCore
LoRa mesh pipeline.

Voice recording UI (long-press send button or + menu) is gated behind
Platform.isIOS || Platform.isMacOS since the `record` package only
supports microphone capture on those platforms in this build.

Key changes:
- VoiceRecorderService: streams 8kHz mono PCM chunks via `record` package
- VoicePlayerService: decodes Codec2 bytes to WAV and plays via audioplayers
- VoiceCodecService: async Codec2 encode/decode in background isolates
- VoiceProvider: reassembles multi-packet sessions, drives playback
- VoiceMessageBubble: shows packet progress, play/stop controls
- MessagesProvider: detects V: prefix, routes to VoiceProvider
- MessagesTab: PTT long-press gesture + recording indicator (iOS/macOS)
- Message model: isVoice + voiceId fields for session tracking
- Auto-selects codec mode from radio bandwidth (700C/1200/1300 bps)
- codec2_flutter + meshcore_client switched from path to git deps

lib/providers/messages_provider.dart

@@ -8,6 +8,7 @@ import '../services/message_storage_service.dart';
 import '../services/notification_service.dart';
 import '../utils/sar_message_parser.dart';
 import '../utils/drawing_message_parser.dart';
+import '../utils/voice_message_parser.dart';
 import '../l10n/app_localizations.dart';
 import 'helpers/message_retry_manager.dart';
 
@@ -277,6 +278,17 @@ class MessagesProvider with ChangeNotifier {
       );
     }
 
+    // Check if it's a voice message (V:...) and not already marked
+    if (VoicePacket.isVoiceText(enhancedMessage.text) && !enhancedMessage.isVoice) {
+      final pkt = VoicePacket.tryParseText(enhancedMessage.text);
+      if (pkt != null) {
+        enhancedMessage = enhancedMessage.copyWith(
+          isVoice: true,
+          voiceId: pkt.sessionId,
+        );
+      }
+    }
+
     // For channel messages with sender name, try to link with contact
     Message finalMessage = enhancedMessage;
     if (enhancedMessage.isChannelMessage &&