feat: add model management module with provider CRUD

- New /models page with provider list (built-in + custom)
- Add provider via preset selection or custom URL with auto-fetch models
- Delete provider removes from auth.json credential_pool + config.yaml custom_providers
- Auto-switch model on add, fallback switch on delete
- Sync sidebar ModelSelector on all provider changes
- Unified provider presets in shared/providers.ts (frontend + backend)
- Backend uses hardcoded catalog first, live probe as fallback

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

src/api/system.ts

@@ -49,6 +49,7 @@ export interface CustomProvider {
   base_url: string
   api_key: string
   model: string
+  providerKey?: string | null
 }
 
 export async function checkHealth(): Promise<HealthResponse> {