refactor: refactor the model configuration page in agentscope-desktop

### Is your feature request related to a problem?

_No response_

### Describe the Solution You'd Like

To better organize the API providers and model configurations, we should introduce a two-level structure — with the provider as the first level and models as the second level. This way, metadata such as whether a model supports multimodal capabilities can be pre-defined within the configuration. It will make things much easier when we add multimodal support in the future.

### Describe Alternatives You've Considered

_No response_

### Additional Context

_No response_

### Checklist

- [x] I have searched existing issues and confirmed this is not a duplicate