fix(voice): fix silent STT failure — use AAC/M4A, store onError, guard NaN amplitude

Three bugs causing silent STT failure on Android:

1. AudioEncoder.opus produces an OGG container on Android but the file
   was named .webm — faster-whisper rejected it due to format mismatch.
   Changed to AudioEncoder.aacLc + .m4a (reliable on all Android versions).
   Updated voice_api.dart to send audio/mp4 MIME type accordingly.

2. onError callback was never stored, so errors in _startListening() and
   _handleSilence() were silently swallowed. Now stored as _onError and
   called before exitVoiceMode() on any failure.

3. Amplitude stream can emit NaN or ±Infinity on some Android devices
   during recorder initialisation. NaN < -40.0 is false, so silence was
   never detected. Now treated explicitly as silence.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

lib/providers/voice_provider.dart

@@ -100,6 +100,7 @@ class VoiceNotifier extends Notifier<VoiceState> {
 
   // Voice mode callbacks
   Future<void> Function(String transcript)? _onTranscript;
+  void Function(String message)? _onError;
   bool _enableTts = false;
 
   // Streaming TTS state
@@ -165,6 +166,7 @@ class VoiceNotifier extends Notifier<VoiceState> {
     }
 
     _onTranscript = onTranscript;
+    _onError = onError;
     _enableTts = enableTts;
     _tempDir = await getTemporaryDirectory();
 
@@ -184,6 +186,7 @@ class VoiceNotifier extends Notifier<VoiceState> {
     _lastSeenLength = 0;
     _streamComplete = false;
     _onTranscript = null;
+    _onError = null;
     state = const VoiceState();
   }
 
@@ -217,18 +220,25 @@ class VoiceNotifier extends Notifier<VoiceState> {
     state = state.copyWith(mode: VoiceMode.recording);
 
     final dir = _tempDir ?? await getTemporaryDirectory();
+    // AAC/M4A is reliably supported on Android (unlike WebM/Opus which can
+    // produce OGG bytes in a .webm file, confusing server-side decoders).
     final path =
-        '${dir.path}/voice_rec_${DateTime.now().millisecondsSinceEpoch}.webm';
+        '${dir.path}/voice_rec_${DateTime.now().millisecondsSinceEpoch}.m4a';
 
-    await _recorder!.start(
-      const RecordConfig(encoder: AudioEncoder.opus, sampleRate: 16000),
-      path: path,
-    );
+    try {
+      await _recorder!.start(
+        const RecordConfig(encoder: AudioEncoder.aacLc, sampleRate: 16000),
+        path: path,
+      );
 
-    _amplitudeSubscription?.cancel();
-    _amplitudeSubscription = _recorder!
-        .onAmplitudeChanged(const Duration(milliseconds: 200))
-        .listen(_onAmplitude);
+      _amplitudeSubscription?.cancel();
+      _amplitudeSubscription = _recorder!
+          .onAmplitudeChanged(const Duration(milliseconds: 200))
+          .listen(_onAmplitude);
+    } catch (e) {
+      _onError?.call('Microphone error: could not start recording');
+      exitVoiceMode();
+    }
   }
 
   void _onAmplitude(Amplitude event) {
@@ -238,7 +248,12 @@ class VoiceNotifier extends Notifier<VoiceState> {
         DateTime.now().millisecondsSinceEpoch - _recordingStartMs;
     if (elapsed < _minRecordingMs) return;
 
-    if (event.current < _silenceThresholdDb) {
+    final db = event.current;
+    // Guard against NaN / ±Infinity which can arrive on some Android devices
+    // when the recorder is initialising. Treat invalid readings as silence.
+    final isSilent = db.isNaN || db.isInfinite || db < _silenceThresholdDb;
+
+    if (isSilent) {
       _silenceMs += 200;
       if (_silenceMs >= _silenceDurationMs) {
         _amplitudeSubscription?.cancel();
@@ -289,8 +304,8 @@ class VoiceNotifier extends Notifier<VoiceState> {
       if (!_enableTts && state.voiceModeActive) {
         await _startListening();
       }
-    } catch (_) {
-      // Network/API error — exit voice mode
+    } catch (e) {
+      _onError?.call('Voice error: transcription failed');
       exitVoiceMode();
     }
   }