Improve transcription accuracy with overlapping audio chunks

Changes:
1. Changed UI text from "Recording" to "Transcribing" for clarity
2. Implemented overlapping audio chunks to prevent word cutoff

Audio Overlap Feature:
- Added overlap_duration parameter (default: 0.5 seconds)
- Audio chunks now overlap by 0.5s to capture words at boundaries
- Prevents missed words when chunks are processed separately
- Configurable via audio.overlap_duration in config.yaml

How it works:
- Each 3-second chunk includes 0.5s from the previous chunk
- Buffer advances by (chunk_size - overlap_size) instead of full chunk
- Ensures words at chunk boundaries are captured in at least one chunk
- No duplicate transcription due to Whisper's context handling

Example with 3s chunks and 0.5s overlap:
  Chunk 1: [0.0s - 3.0s]
  Chunk 2: [2.5s - 5.5s]  <- 0.5s overlap
  Chunk 3: [5.0s - 8.0s]  <- 0.5s overlap

Files modified:
- client/audio_capture.py: Implemented overlapping buffer logic
- config/default_config.yaml: Added overlap_duration setting
- gui/main_window_qt.py: Updated UI text, passed overlap param
- main_cli.py: Passed overlap param

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

gui/main_window_qt.py

@@ -278,6 +278,7 @@ class MainWindow(QMainWindow):
             self.audio_capture = AudioCapture(
                 sample_rate=self.config.get('audio.sample_rate', 16000),
                 chunk_duration=self.config.get('audio.chunk_duration', 3.0),
+                overlap_duration=self.config.get('audio.overlap_duration', 0.5),
                 device=audio_device
             )
 
@@ -296,7 +297,7 @@ class MainWindow(QMainWindow):
             self.is_transcribing = True
             self.start_button.setText("⏸ Stop Transcription")
             self.start_button.setStyleSheet("background-color: #e74c3c; color: white;")
-            self.status_label.setText("🔴 Recording...")
+            self.status_label.setText("🔴 Transcribing...")
 
         except Exception as e:
             QMessageBox.critical(self, "Error", f"Failed to start transcription:\n{e}")