- Add ProviderCategory type with official, cn_official, aggregator, third_party, and custom categories
- Update Provider interface and Rust struct to include optional category field
- Enhance ProviderForm to automatically sync category when selecting presets
- Improve PresetSelector to show category-based styling and hints
- Add category classification to all provider presets
- Support differentiated interactions (e.g., hide API key input for official providers)
- Maintain backward compatibility with existing configurations