fix(voice): eliminate dual-recorder conflict and defunct element crashes

VadHandler is now the sole mic owner — removed separate AudioRecorder that
caused audio focus contention on Android. Audio from onSpeechEnd is encoded
as WAV for Whisper. Provider switched to autoDispose to match widget lifecycle,
preventing defunct element assertions. Recording UI deferred until mic is open.

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

lib/data/api/voice_api.dart

@@ -40,7 +40,7 @@ class VoiceApi {
     }
   }
 
-  /// POST WebM/Opus audio bytes and return the transcript string.
+  /// POST audio bytes (WAV) and return the transcript string.
   /// [context] is optional recent conversation text passed as initial_prompt
   /// to Whisper, reducing mishearings of domain-specific words.
   /// Returns empty string on empty or error response.
@@ -49,8 +49,8 @@ class VoiceApi {
       final fields = <String, dynamic>{
         'audio': MultipartFile.fromBytes(
           audioBytes,
-          filename: 'audio.m4a',
-          contentType: DioMediaType('audio', 'mp4'),
+          filename: 'audio.wav',
+          contentType: DioMediaType('audio', 'wav'),
         ),
         if (context != null && context.isNotEmpty) 'context': context,
       };