feat: Add CPU/GPU Device Indicator to GUI

[dziki]

Summary

This PR adds a visual indicator to the main application window that explicitly displays whether the Whisper model is running on the CPU or GPU.

Previously, users had to rely on console logs or system performance monitors to confirm if CUDA acceleration was active. Now, the interface provides immediate, color-coded feedback next to the model name.

Key Changes:

• core_logic.py:
  • Added a device_changed signal to the DictationWorker class.
  • Updated load_model to emit "GPU" or "CPU" based on the device determined by faster-whisper.
• main_gui.py:
  • Added a device_display_label to the header layout.
  • Implemented update_device_label slot to update the text and color (green for GPU, orange for CPU).