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/voice_provider.dart

@@ -0,0 +1,121 @@
+import 'dart:async';
+import 'package:flutter/foundation.dart';
+import '../utils/voice_message_parser.dart';
+import '../services/voice_codec_service.dart';
+import '../services/voice_player_service.dart';
+
+/// Reassembly state for one voice session.
+class VoiceSession {
+  final String sessionId;
+  final VoicePacketMode mode;
+  final int total;
+  final List<VoicePacket?> packets; // indexed by packet.index
+
+  VoiceSession({
+    required this.sessionId,
+    required this.mode,
+    required this.total,
+  }) : packets = List.filled(total, null);
+
+  int get receivedCount => packets.where((p) => p != null).length;
+  bool get isComplete => receivedCount == total;
+
+  /// Total estimated audio duration in seconds (sum of all received packets).
+  double get estimatedDurationSeconds {
+    var ms = 0;
+    for (final p in packets) {
+      if (p != null) ms += p.durationMs;
+    }
+    return ms / 1000.0;
+  }
+}
+
+/// Manages incoming voice packet sessions and coordinates playback.
+class VoiceProvider with ChangeNotifier {
+  final VoiceCodecService _codec;
+  final VoicePlayerService _player;
+
+  /// Active sessions keyed by sessionId.
+  final Map<String, VoiceSession> _sessions = {};
+
+  /// Currently playing session ID, or null.
+  String? _playingSessionId;
+
+  VoiceProvider({
+    required VoiceCodecService codec,
+    required VoicePlayerService player,
+  })  : _codec = codec,
+        _player = player;
+
+  // ── Session accessors ────────────────────────────────────────────────────
+
+  VoiceSession? session(String sessionId) => _sessions[sessionId];
+  bool isComplete(String sessionId) => _sessions[sessionId]?.isComplete ?? false;
+  bool isPlaying(String sessionId) =>
+      _playingSessionId == sessionId && _player.isPlaying;
+
+  // ── Packet reception ─────────────────────────────────────────────────────
+
+  /// Add an incoming [packet] to its session.  Creates the session on first packet.
+  /// Returns true if the session just became complete.
+  bool addPacket(VoicePacket packet) {
+    _sessions.putIfAbsent(
+      packet.sessionId,
+      () => VoiceSession(
+        sessionId: packet.sessionId,
+        mode: packet.mode,
+        total: packet.total,
+      ),
+    );
+
+    final session = _sessions[packet.sessionId]!;
+    if (packet.index < session.total) {
+      session.packets[packet.index] = packet;
+    }
+
+    final justComplete = session.isComplete;
+    notifyListeners();
+    return justComplete;
+  }
+
+  // ── Playback ─────────────────────────────────────────────────────────────
+
+  /// Decode and play the voice session with [sessionId].
+  /// Plays whatever packets are available (handles partial reception gracefully).
+  Future<void> play(String sessionId) async {
+    final session = _sessions[sessionId];
+    if (session == null) {
+      debugPrint('❌ [VoiceProvider] play($sessionId) — session not found, known: ${_sessions.keys.toList()}');
+      return;
+    }
+
+    debugPrint('🎙️ [VoiceProvider] play($sessionId): ${session.receivedCount}/${session.total} packets, mode=${session.mode.label}');
+
+    try {
+      final pcm = await _codec.decodePackets(session.packets, session.mode);
+      debugPrint('🎙️ [VoiceProvider] decoded ${pcm.length} PCM samples');
+      _playingSessionId = sessionId;
+      notifyListeners();
+      await _player.play(pcm);
+    } catch (e, st) {
+      debugPrint('❌ [VoiceProvider] Playback error: $e\n$st');
+    } finally {
+      if (_playingSessionId == sessionId) {
+        _playingSessionId = null;
+        notifyListeners();
+      }
+    }
+  }
+
+  Future<void> stop() async {
+    await _player.stop();
+    _playingSessionId = null;
+    notifyListeners();
+  }
+
+  @override
+  void dispose() {
+    _player.dispose();
+    super.dispose();
+  }
+}