Function migrateTTSCookies

Migrate TTS settings from cookies to localStorage.

Old cookie format:

• tts[{lang}Voice] - voice name
• tts[{lang}Rate] - rate value
• tts[{lang}Pitch] - pitch value
• tts[{lang}RegName] - alternative voice name key

This function detects existing TTS cookies, migrates them to localStorage, and removes the old cookies.