Added 60dB integration

[manishEMS47]

What we added (client side, in this repo):

1. src/tts/SpeechModel.ts — case "60dB" so 60dB voices route like ElevenLabs/OpenAI.
2. public/icons/logos/60db.svg — placeholder logo for the voice menu.
3. test/tts/AudioProviders.spec.ts — test for 60dB routing (6/6 passing).
4. doc/60db-integration.md — backend contract spec.
5. src/tts/README.md — "Audio engines" section.

How to use it:

• User: once the backend serves 60dB voices, they show up in the same TTS voice picker (with the 60dB logo) — pick one and replies
  are read aloud by 60dB. No new UI.
• You (to go live): implement the api.saypi.ai backend per doc/60db-integration.md — map api.60db.ai/myvoices → /voices with
  powered_by:"60dB", proxy TTS via 60dB's WebSocket, route /transcribe → api.60db.ai/stt. Keep the literal exactly "60dB".

[rosscado]

This sounds interesting @manishEMS47 . I see that 60dB have excellent Hindi voices.
Which voices do you suggest I add?
What platform do you want them for - pi/claude/chatgpt/all?