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/screens/chat/chat_screen.dart

@@ -68,8 +68,6 @@ class _ChatScreenState extends ConsumerState<ChatScreen>
     WidgetsBinding.instance.removeObserver(this);
     _controller.dispose();
     _scrollController.dispose();
-    // Exit voice mode if the user navigates away mid-session.
-    ref.read(voiceProvider.notifier).exitVoiceMode();
     super.dispose();
   }