Add Synthorai as a supported provider

Is your feature request related to a problem? Please describe. 新功能是否与解决某个问题相关, 请描述

I'd like to use Synthorai (https://synthorai.io) as a model source in chatGPTBox, similar to the existing custom-API-key providers.

Describe the solution you'd like 你期望的新功能实现方案

Add Synthorai as a supported provider option. Synthorai is an OpenAI/Anthropic-compatible LLM gateway routing to 113 models across 11 upstream providers (Claude, GPT, Gemini, GLM, Kimi, DeepSeek, Qwen, etc.) at direct upstream pricing, no markup. API docs: https://synthorai.io/docs, base URL https://synthorai.io, standard Bearer auth.

Additional context 其他

Free tier for testing: 10 calls / up to $1, no card required. Happy to help with a PR if useful.