feat(stt): pass last assistant response as Whisper context to reduce mishearings

lib/data/api/voice_api.dart

@@ -40,16 +40,20 @@ class VoiceApi {
   }
 
   /// POST WebM/Opus audio bytes 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.
-  Future<String> transcribe(Uint8List audioBytes) async {
+  Future<String> transcribe(Uint8List audioBytes, {String? context}) async {
     try {
-      final formData = FormData.fromMap({
+      final fields = <String, dynamic>{
         'audio': MultipartFile.fromBytes(
           audioBytes,
           filename: 'audio.m4a',
           contentType: DioMediaType('audio', 'mp4'),
         ),
-      });
+        if (context != null && context.isNotEmpty) 'context': context,
+      };
+      final formData = FormData.fromMap(fields);
       final response = await _dio.post(
         '/api/voice/transcribe',
         data: formData,