fix(auth): use correct model config format (object, not string)

OpenClaw expects agents.defaults.model as { primary: "provider/model" }
not a plain string. The previous code wrote a string which caused:
"agents.defaults.model: Invalid input: expected object, received string"

electron/utils/openclaw-auth.ts

@@ -197,9 +197,10 @@ export function setOpenClawDefaultModel(provider: string): void {
   }
   
   // Set the default model for the agents
+  // model must be an object: { primary: "provider/model", fallbacks?: [] }
   const agents = (config.agents || {}) as Record<string, unknown>;
   const defaults = (agents.defaults || {}) as Record<string, unknown>;
-  defaults.model = model;
+  defaults.model = { primary: model };
   agents.defaults = defaults;
   config.agents = agents;