Feature: Add Flutter Voice Selection UI and Platform Channels for DSP Control

### 🚀 The feature

Add Flutter UI components for selecting predefined voice profiles and toggling preview mode.
Introduce platform channels to communicate user selections (e.g., selected voice profile, preview enabled/disabled) from Flutter to native code.

Flutter will strictly act as a control layer and will not process or handle audio buffers.

### Motivation, pitch

Users need a simple and intuitive way to select voice styles before or during a session. Flutter is the appropriate layer for user interaction, while native code handles audio processing.

By keeping Flutter limited to UI and control signals via platform channels, we maintain real-time performance and avoid running audio logic in Dart. This preserves responsiveness and ensures clean separation between UI and audio processing.