Fix #219: Suppress [BLANK_AUDIO] tokens in whisper.cpp output (#222)

- Add suppress_blank=True and suppress_non_speech_tokens=True parameters
  to whisper.cpp Model initialization to prevent blank audio tokens
  from being output when transcribing silent audio
- This implements Option 2 from the investigation: suppress at the
  whisper.cpp level rather than post-processing filtering
- Root cause: whisper.cpp outputs a special [BLANK_AUDIO] token
  when transcribing silent audio, which was being inserted into the
  text field when users paused with the mic on

src/vocalinux/speech_recognition/recognition_manager.py

@@ -633,7 +633,12 @@ class SpeechRecognitionManager:
             cpu_count = multiprocessing.cpu_count()
 
             load_start_time = time.time()
-            self.model = Model(model_path, n_threads=n_threads)
+            self.model = Model(
+                model_path,
+                n_threads=n_threads,
+                suppress_blank=True,
+                suppress_non_speech_tokens=True,
+            )
             load_duration = time.time() - load_start_time
 
             logger.info(