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

pubspec.yaml

@@ -44,7 +44,18 @@ dependencies:
   meshcore_client:
     git:
       url: https://github.com/dz0ny/meshcore_client.git
-      ref: main
+
+  # Codec2 ultra-low-bitrate speech codec (FFI plugin)
+  codec2_flutter:
+    git:
+      url: https://github.com/dz0ny/codec2_flutter.git
+  ffi: ^2.1.0
+
+  # Voice recording (raw PCM stream at 8000 Hz)
+  record: ^6.0.0
+
+  # Voice playback (plays WAV from file)
+  audioplayers: ^6.0.0
 
   # BLE connectivity
   flutter_blue_plus: ^2.0.0