* feat(tui): add /provider command, custom registry import, and tabbed model selector
- add "/provider" slash command for managing AI providers with CRUD UI
- add custom registry import via api.json URL and Bearer token
- introduce tabbed model selector grouped by provider
- add fetchCustomRegistry and applyCustomRegistryProvider in oauth package
- replace deprecated "/connect" command with unified "/provider" flow
- update provider and slash-command documentation
* feat(tui): background provider model refresh at startup
- add `refreshAllProviderModels` utility supporting managed OAuth, open platforms and custom registries
- wire background refresh into `KimiTUI` startup via `AuthFlowController`
- add `providerSwitchHint` option to `ModelSelector` and enable it in `TabbedModelSelector`
- update `TabbedModelSelector` hint wording from "switch" to "provider"
