feat: improve WebUI usability with presets and native dialogs

[GRay525]

This PR introduces usability improvements to the GPT-SoVITS WebUI.

Changes:

1. Fixed Hardcoded Default Paths

• Replaced developer-specific paths (e.g., D:\GPT-SoVITS\raw\xxx) with empty strings
• Added helpful placeholder text for path inputs

2. Added Inference Parameter Preset System

• New preset dropdown: Fast Synthesis / High Quality / Balanced / Custom
• One-click configuration for common use cases

3. Native File/Folder Selection Dialogs

• Replaced confusing file upload components with native tkinter dialogs
• Added clear buttons: 📁 Select Folder and 📄 Select File

4. Fixed Missing i18n Labels

• Added English translations for new UI elements

Files Changed

• webui.py
• GPT_SoVITS/inference_webui_fast.py
• api.py
• tools/i18n/locale/en_US.json

Backward Compatibility
✅ All changes are backward compatible. No breaking changes.