phsl / new-api

- Introduced a new `FetchModels` endpoint to retrieve model IDs from a specified base URL and API key, enhancing flexibility for different channel types.
- Refactored `FetchUpstreamModels` to simplify base URL handling and improve error messages during response parsing.
- Updated API routes to include the new endpoint and adjusted the frontend to utilize the new fetch mechanism for model lists.
- Removed outdated checks for channel type in the frontend, streamlining the model fetching process.