[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.

Start by reading the existing OpenAI-compatible bot integrations and the project’s supported-bot registration path, then compare their request configuration with Synthorai’s API documentation. Verify that Synthorai can be selected in ChatALL, uses the documented Bearer authentication and base URL, and returns responses through the normal comparison flow.

Written by the indexing model from the issue text.