- RpcCommand: add get_login_providers and login { providerId }
- RpcResponse: add typed responses for both commands
- RpcExtensionUIRequest: add open_url { url, instructions } event
(fire-and-forget; host opens the URL in system browser)
- rpc-mode: handle get_login_providers (returns provider list with
per-provider authenticated status) and login (drives authStorage.login
using RpcExtensionUIContext for onPrompt dialogs and notify for
onProgress; emits open_url for the auth page URL)
- rpc-client: add getLoginProviders() and login(providerId) methods