Add support for spanish voice generation

[mpast]

Add Spanish Voice Support to Autiobooks

Overview

This PR enables Spanish language support for Autiobooks, allowing users to generate audiobooks using Spanish voices from the Kokoro TTS model. The changes include enabling Spanish voices in the UI and fixing a critical bug in the TTS pipeline that was preventing proper language code detection.

Problem

While Kokoro TTS supported Spanish, Autiobooks only exposed English voices (American and British).

Testing

To verify Spanish support works:

1. Launch Autiobooks GUI
2. Look for voices with 🇪🇸 flag in the voice dropdown
3. Select a Spanish voice (e.g., em_santa)
4. Select an EPUB file
5. Click "Convert epub"
6. Verify audio generates properly with Spanish pronunciation