feat: Enforce exclusive auto-request (one client at a time)

Server is now source of truth for autoRequest. When a client enables it,
all other clients lose it. Broadcast includes autoRequest per client,
frontend syncs from server state on each torch-update.

frontend/src/stores/torch.ts

@@ -4,6 +4,7 @@ import { ref, computed } from 'vue'
 export interface TorchClient {
   id: string
   name: string
+  autoRequest: boolean
   userAgent: string
   hostname: string
   url: string