fix: provider-aware model selection to prevent cross-provider conflicts

When multiple providers share the same model name, the selector now
uses both model ID and provider as the unique identifier instead of
model name alone. Backend returns default_provider alongside default
model, and model switching sends provider to the config.

Fixes #52

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

packages/client/src/api/hermes/system.ts

@@ -33,6 +33,7 @@ export interface AvailableModelGroup {
 
 export interface AvailableModelsResponse {
   default: string
+  default_provider: string
   groups: AvailableModelGroup[]
 }