[FEAT] Add Synthorai as a supported bot

### Is your feature request related to a problem? / 你想要的功能和什么问题相关？

I'd like to compare responses from Synthorai (https://synthorai.io) alongside other models in ChatALL, but it's not currently in the supported bot list.

### Describe the solution you'd like. / 你想要的解决方案是什么？

Add Synthorai as a supported bot, following the same pattern as the existing OpenAI-compatible integrations. 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.

### Describe alternatives you've considered. / 你考虑过的其他方案是什么？

N/A — no generic custom-endpoint option currently in ChatALL as far as I could find, only named bots.

### Additional context / 其他信息

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