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/main.dart

@@ -11,7 +11,10 @@ import 'providers/messages_provider.dart';
 import 'providers/map_provider.dart';
 import 'providers/drawing_provider.dart';
 import 'providers/channels_provider.dart';
+import 'providers/voice_provider.dart';
 import 'providers/app_provider.dart';
+import 'services/voice_codec_service.dart';
+import 'services/voice_player_service.dart';
 import 'services/tile_cache_service.dart';
 import 'services/notification_service.dart';
 import 'services/locale_preferences.dart';
@@ -231,10 +234,19 @@ class _MeshCoreSarAppState extends State<MeshCoreSarApp> {
         ),
         ChangeNotifierProvider(create: (_) => ChannelsProvider()),
 
+        // Voice provider (packet reassembly + playback)
+        ChangeNotifierProvider(
+          create: (_) => VoiceProvider(
+            codec: VoiceCodecService(),
+            player: VoicePlayerService(),
+          ),
+        ),
+
         // Tile cache service
         Provider(create: (_) => TileCacheService()),
 
         // App provider that coordinates everything
+        // VoiceProvider is read via context.read inside create since it's already registered above
         ChangeNotifierProxyProvider6<
           ConnectionProvider,
           ContactsProvider,
@@ -250,6 +262,7 @@ class _MeshCoreSarAppState extends State<MeshCoreSarApp> {
             messagesProvider: context.read<MessagesProvider>(),
             drawingProvider: context.read<DrawingProvider>(),
             channelsProvider: context.read<ChannelsProvider>(),
+            voiceProvider: context.read<VoiceProvider>(),
             tileCacheService: context.read<TileCacheService>(),
           ),
           update:
@@ -270,6 +283,7 @@ class _MeshCoreSarAppState extends State<MeshCoreSarApp> {
                     messagesProvider: messages,
                     drawingProvider: drawings,
                     channelsProvider: channels,
+                    voiceProvider: context.read<VoiceProvider>(),
                     tileCacheService: tileCache,
                   ),
         ),